I also wouldn't mind seeing him take the next step forward and put a bit more into the stage backgrounds. Having different types of enemies that are integrated into the backgrounds (yes, like an arcade shooter) would bring so much more life into the stages and make them a lot more interesting. Currently I just find them so abstract and usually unmemorable. Stage 4 of MoF is one of my favorites in the series simply because it has enemies emerging out from behind a waterfall.
I actually like this a lot. I have bad/weird eyes with notoriously bad contrast sensitivity, and a lot of shmup backgrounds are so goddamn noisy that my eyes at times stop being able to differentiate between bullets, enemies, portions of the background that *won't* kill you for a change, and medals/point items/powerups/1ups/bomb items/etc. It all blends together into this weird mess. With the Touhou backgrounds, that's not an issue, they're usually very simple and made predominantly of solid colours, and thus it's virtually always easy to see when something is moving overtop of the damn things and if it's a bullet or a 1up/point item/power item/etc.
Other things that I like:
THE PATTERNS. Probably the biggest draw of Touhou for me is the complex patterns that boss spellcards have that add a puzzlegame aspect to the shmup gameplay.
Fairly consistently good gameplay, for that matter.
The amount of lives is nice, warrants less grinding for a 1cc while still letting you pull off stuff like No Bombs 1ccs if you really want to put a crapton of time into playing the games.
The music. The music is simply excellent.
For that matter, the synchronization of the games to the music - this is something that needs to be done more often in shmups, it alone makes playing a given stage approximately 10x more awesome for me.
Dislikes:
Virtually all of the fanbase. Jesus Arc-Welding Christ, the fandumb just gets to ludicrous extents half of the time.
BOSS MILK/STAGE MILK-based scoring systems. Blargh. And people wonder why I don't play Touhou for score at all :V
UFO curvylasers. What was ZUN drinking when he designed those?
UFO's power system, for that matter. Losing a whole 1.xx power upon death? What the shit, way to make recovering from a death in UFO like 10x harder for
absolutely no good reason, ZUN.
Speaking of fandumb, virtually all of the memes, but that's neither here nor there.
Indifferent:
Characters, plot, doujins. Meh. Really couldn't care less about the vast majority of this stuff, it isn't terribly interesting. Especially the doujins, which I find are all terribly boring and usually also fairly asinine, tbqh.