WARNING: VERY EXTENSIVE.
The very first thing that caught my attention was ZUN's art; it's better than before. The drawings are still a bit weird, but now they are much better than when he first started. It goes to show how dedication gradually improves skill.
While I'm on the skill talk, let me tell you right now that Ten Desires is actually a pretty easy game. To the point that if it used the same system of EoSD and MoF (points = Lives; set lives at your whim in EoSD), I would call it the easiest one yet without hesitation.
The primary reason is that the bullet patterns are simply not as hard as seen in previous games. The difficulty starts picking up, predictably, at stage 4, but unlike some Touhou games, the Stage 4 boss won't change her patterns or Spellcards depending on the character you choose, you can choose whomever you like to beat her with and practice the hell out of her.
Next up is the new mechanic to this game; Trance. The Trance state can be surmised as a period of momentary godhood. To enter Trance, you first need to fill up at least one of the three Trance slots by collecting blue spirits. You can only activate Trance voluntarily when you have all three slots filled. Otherwise, you can only use Trance when you have any less than three slots and dying, in which case you will use up all your Trance before actually dying.
When in Trance, you're completely invincible, your shot changes and does a LOT more damage in a rapid fire state, and all the spirits that manifest from defeating enemies become score points rather than charging your Trance. This means that it's best to use Trance when you have loads of spirits onscreen, or when you know a tough section is coming up and you don't want to waste bombs.
For people with slow reflexes, Trance will seem like a godsend, but to veterans and those with fast reflexes, there is a terrible flaw that does not become immediately apparent; any time you have one or more Trance slots filled, you can't deathbomb. As said before, getting hit when you have at least one slot automatically induces Trance with whatever Trance you have before you die. Because the Trance is used just as you get hit, there is no window for deathbombing. This means that you must be careful once your Trance is nearly filled up, as a stray bullet is all that is needed to lose a life and perhaps a couple of bombs you were saving for later.
By the way, using Trance is functionally counted as the same thing as using a bomb by the game; therefore, Trancing during a Spell card counts as a fail. Moreover, being in Trance AT ANY TIME during a Spellcard counts as a fail; so if you Tranced during a non-Spell and were still in Trance when the Spell began, you automatically fail it.
EDIT (4/17/2012). Messing around with the Trance system, I found another use for it; like in SA and UFO, you have to assemble your lives and bombs with parts. In this game, while in Trance, life and bomb parts are worth double. This means that in order to maximize your number of available bombs and lives, you have to deliberately hold out on Trancing until you get to a part where several parts are dropped before doing it. This helps mitigate the insane life part requirements later on (16 parts for 1 life? WTF?) Since the game's been out for quite a while now, this should be old news.
Also, spirits appear much more frequently the closer you are to the enemy as you shoot her, meaning if you want to score big (and get more Trance for points/lives/bombs), you HAVE to take the risk.
Leaving Trance behind, lets foucs on our story and characters, hmm?
There are four characters you can use: Reimu, Marisa, Sanae (what happened to Sakuya? =( ) and Youmu. However, to make up for the several options, each one only has on style, so we can safely classify each one.
Reimu Hakurei, as expected, is the homing type. Her homing shot type gives you good range, but their not very strong on their own. Fortunately, focusing swaps homing for full frontal rapid fire, which deals good damage. Together with her small hitbox, this means you have the easiest time dodging bullets with her. Her bomb is the classic Fantasy Seal, with its usual homing attack.
Marisa Kirisame, as is her style, is the power type. Her normal shots are lasers. For the first two levels, they're just frontal lasers, but for levels 3 and 4, the lasers are angled, so that means Marisa now has a pretty good range of fire. Coupled with her speed, Marisa can now rapidly sweep the screen of weak enemies. Her focus shot changes the lasers to missiles, similar to Nitori's style in SA. The missiles first have to hit their target, unlike the lasers which instantly and constantly hit their target, but they make up for their increased damage and small splash effect. Her bomb is the Master Spark, which can be angled by moving. You can also use your shot as you're firing the Master Spark for a deadly combination.
Sanae Kochiya's style is the "wide" type or "range" type (damn you for stealing Sakuya's spot!). In a sort of weird compromise between Reimu and Marisa, Sanae shoots waves that have good range and deal decent damage, but focusing sacrifices the range for waves that curve inwards, increasing damage output. Not much to say here. Her bomb (which I don't know the name of, sorry) causes a spinning star to form around her. You may now fly around the stage to clear bullets from the screen as you please.
Lastly we have Youmu Konpaku, who has the "weird" type, as you could call it. Youmou's regular shot is similar to her IN one, in where it shoots out in a straight line before fanning out at the end. But rather than having Myon shooting contrary to her movement, there are (up to four) options that trail behind her, following her movements and shooting in tandem (essentially the same as one of Marisa's styles in MoF). When you focus with Youmu is when you notice why I call this the "weird" type: Youmu's Focus shot is a charged attack. Holding the fire button when in Focus mode charges up energy. Once the charging sound ends, you can release the button. Youmu will use Roukan-kan (her sword) to slash multiple times in wide lines in front of her. Max Power Youmu's attack has even more range.
This attack does very high damage, has a very good range and does not take long to charge, making it essentially an easy-modification for certain parts. Why only certain parts? The attack can clear stronger fairies and groups of smaller ones when they appear before they even get a chance to attack, making thins a lot easier on you. However, some spellcards will prove to be very troublesome as Youmu's unorthodox regular shot is cumbersome for effectively capturing them, meaning you have to rely on the charged attack, and then you'd have to worry about dodging bulets, charging the attack and releasing when it's done, keeping an eye on the bullets further up, the boss's position...
Her bomb (of which, again, I don't know its name) causes multiple large slashes to appear across the screen. Note that only bullets touched by the slashes are cleared.
I'd say my favorites are Marisa's and Youmu's styles (partly because they're both pretty awesome here, partly because I don't like Sanae and especially Reimu all that much), as they're both pretty powerful styles when mastered.
...Let's make the story part very short in order to close up quickly.
Basically, our heroine notices a proliferation of spirits appearing in Gensokyo, so she goes to Hakyogyokurou to ask about it (Youmu, being from the place, instead notices spirits heading for the human village). Encountering Yuyuko, the heroine asks about it, but she only agrees to talk if the heroine beats her (Youmu gets the same treatment, and she notes how ridiculous Yuyuko sounds). After the spar, Yuyuko leaves a not-so subtle clue: Myouren Temple. On the way to the temple, they encounter Kyouko Kasodani, who has recently converted to Buddhism and works at the temple.
After the fight, the heroine proceeds to the graveyard, where they run in with Kogasa, who asks for help in dealing with a girl who isn't supposed to be there. The heroine wryly muses about the irony in a yokai asking a human for help in dealing with another yokai, before agreeing to help... after beating Kogasa, that is. (Youmu, having never met Kogasa before, is instead challenged by Kogasa as she wanted to see if Youmu could deal with the girl). Moving on, the heroine encounters Yoshika Miyako, a Jiang-shi (chinese vampire) with very zombie-like atributes due to Jiang-shi becoming less popular in favor of zombies in the outside world. Stating that she cannot allow anyone to pass, the heroine defeats her and proceeds beyond where she stood guard.
Infiltrating a tunnel, the heroine at the end runs into Seiga Kaku, a hermit who will not allow the heroine to pass the gate she protects, beyond which is sealed someone she once taught Taoism to. Seiga is Yoshika's creator, and so we have a double battle when she calls Yoshika to her. (Yoshika can be killed, but she'll just revive after a while). After dealing with them, the heroine descends alongside a large tower inside the mausoleum she just entered, having a brief run in with Soga no Tojiko's ghost. After more descending, Mononobe no Futo appears, a Taoist who follows the teachings of a person named Miko and who was put into suspended animation by the same person so that she could revive Miko.
After defeating her, the heroine goes to the depeest part of the mausoleum and encounters the final boss, Toyosatomimi no Miko, a saint of legend form ancient Japan who spread Buddhism in order to control the masses while secretly being a Taoist who researched it for immortality. She put herself in suspended animation in secret when the research began to take its toll on her, so that she could be revived later, but Buddhist monks erected a temple in her honor over her grave, preventing her resurrection until the outside world began to deny her existance, at which point the grave was moved to Gensokyo by its own principles. Since she so happened to be a legendary figure, Gensokyo compensated and made her powerful, conveniently enough so for a suitable final boss battle.
...Alright, I think I exagerated. Again. I'll just close and say the game's pretty easy, sans the fact Miko is actually fairly challenging, but pretty fun too.
Overall, Ten Desires is a fairly attractive game, visually, but not the greatest Touhou game if someone comes looking for a hard fight. The music itself in most cases could have been better, though there's no denying the classic ZUN feel. The final boss' theme is suitably epic for a great battle, as is expected.
EDIT (4/17/2012). Apparently, if you beat Miko with 3 or more bombs in your stock (bombs in reserve lives don't count), in Normal or higher, and of course, without continues, you get a "Parallel ending." This means that every heroine should have 2 good endings.
Also, Spellcard Practice is BACK FROM IN, AND there is an extra difficulty for 1 specific spellcard for every non-cameo character that is playable in spellcard practice by capturing that derivation's spellcards in all 4 difficulties. This is the Overdrive spellcard, which is either harder than Lunatic, or WAY harder than the Lunatic counterpart.
ZUN's final appeal to hardcore players in this game? I think so.
Note: If it feels like the music, a fairly important component in Touhou games to be sure, is given a mediocre description or review, its because most of the music didn't catch my attention, unlike practically everything in the previous games. Sad, but true, at least in my opinion. Never fear, there are several great songs here, otherwise it really wouldn't feel like a Touhou game.