I've been studying Japanese for six years or so. I've passed the JLPT Level 2.
Right now I'm probably not going to actively or consistently work on any new projects until I finish up till volume ten of Sengoku. However, if a title really interests me, I may translate a chapter or two.
So what kind of manga interest me? I prefer the stuff for older readers, like seinen. I like plotful stuff with detailed characterization, and detailed art. My favorite genres are: historical, horror, and sci-fi/fantasy.
I'm afraid I'm not that interested in translating manga that are already being translated currently by other groups. That means none of the uber-popular Weekly Shounen Jump titles like Naruto or KHR. Nothing against these titles, I just don't want to reinvent the wheel.
Genres I'm not interested in translating: Harem. Episodic manga. I find gag manga really frustrating to translate because translating jokes is really difficult.
Genres I'm neutral on: Romance is hit or miss with me. Sometimes I like it, sometimes I don't. Slice of life is also hit or miss.
Historical: technically, not manga, but if we count manhua as well, the Ravages of Time.
For horror, Uzumaki.
Sci fi: Probably... Five Star Stories (helluva hard to read, though) or 20th Century Boys.
Multiple translators: If the original translator has given up on a series, I have no problem with it, obviously. Otherwise, unless there is something seriously wrong with the first translation, it seems to be a waste of everyone's time.
Have you read Junji Ito's Yami no Koe? It's a collection of short stories (horror, obviously) and there's only one chapter translated. It doesn't look like it's going to be fully translated anytime soon... or ever. Well, since it's one of your favorite author's works I thought maybe you'd consider working on it.