oh dear...i feel compelled
Do it! lol
Everytime the guys in our room shifted the conversation to Touhou, all I could was listen and try to follow along
dammit, I'll bite.
Touhou started out as a brick breaker type of game, then immediately switched over to a vertical shmup. The initial draw to Touhou included its characters which were rather memorable each in their own right, the music, which most people getting into Touhou gained interest from, and the complexity of the danmaku aspect of the shooter. The reason why there is no anime of Touhou is the game series never got licensed. The characters of Touhou and the world it is in (Gensokyo) is owned by one individual Jun'ya Ota (more commonly known as ZUN). ZUN never wanted Touhou to get licensed and continues to create games and sell them at Comiket and Reitaisai.
I couldn't tell you why Touhou got as popular as it is, but it is now one of the largest phenomenons ever. At Comiket, more than 60% of the circles producing doujin are Touhou related. Touhou even has its own festival Reitaisai where everything Touhou related could be. There is even a tournament style gathering for the various multiplayer Touhou games where games like the fighters, the vs shooters and speed runs and challenges of the games are done. This tournament is called Tohgeki (not to be confused with Tougeki or SBO).
Perhaps one of the reasons why it is as popular as it is could stem from the fact that ZUN has never licensed Touhou and doesn't intend to any time soon. By that, anything that the fans want to do (aside from making an anime or revealing endings to the games...ZUN has specifically requested that the endings not be spoiled) is open to them. As such, you will find hundreds of Touhou based games including:
Touhouvania,
Megamari,
Super Marisa Land, and many many more I could list.
There are a few official manga for Touhou including
"Silent Sinner in Blue",
"Eastern and Little Natural Deity", and
"Inaba of the Moon and Inaba of the Earth". If you are looking for fan doujins, there are literally thousands.
Each character is individually made by ZUN as well as their back story and their theme songs. Even if you don't know the characters, chances are you have heard one of the songs from Touhou or remixes by any of the hundreds of fan music groups.
Each character has their own charm to them and everyone has a favorite character. My two are Rumia and Meiling. With well over 100 characters in the ZUNiverse, there are several scenarios that fans make up for them as there is such a large variety that can be done.
Chances are good, on any online thread, there is at least one person the knows something about Touhou. The mere level of popularity with Touhou is one of the most amazing things about it and why it is so great...because everyone can be a fan of it for some reason.