1.) What fandom do you center on most?
Depends. I get involved in new fandoms all the time, whether I'm dedicating my entire time towards that fandom or otherwise researching and discovering more about it. My major fandoms right now are Firefly, Doctor Who, and Veronica Mars, and newly discovered ones I'm spending much time with are The Suite Life of Zack and Cody, The Tribe, and Battlestar Galactica.
2.) Do you contribute to it much (write fanfiction, draw fanart, participate in online communities and discussions)?
I'm generally just a participant in discussions and browsing about the fandom communities.
3.) Do you think that such things are good or harmful to the fandom and why?
Oh, I hardly think that contributing to a particular fandom in those ways (fanart, fanfiction, discussions, attending conventions and whatnot) would be considered harmful. In fact, I'd think it would actually be helping that fandom. I mean, think about how huge the Fireflyverse has grown since its premature cancellation. It's because of the fan appreciation and contributations that helped the fandom grow into a large community of people who love the same universe Joss created, which helped get the production of Serenity the Big Damn Movie. So no, definitely not harmful at all. Now, if we're talking about an individual that takes advantage or abuses this kind of appreciation, well that's another story altogether.
4.) Do you think its good or harmful for the original creator?
Unless we're talking about the Anne Rice incident where she has prohibited fans from writing fanfiction, then I don't see how harmful it is to the original creator.
5.) Why do you like this fandom in particular?
For Veronica Mars it all comes down to the mysteries, the film noir style and the character of Veronica herself that attracted me to the series in the first place; the pairings and relationships of characters are secondary for me, unlike some folks in that fandom. The new Doctor Who is a brand new fandom, yet I am so enthralled with it because, hello, Ninth Doctor and Rose! For those who are watching the second season with the Tenth Doctor, I haven't gotten that far yet. Also, there's the love for the TARDIS and the creepifying Daleks. With Firefly, well, should there really be an explanation for that one? With my brand new obsessions, The Suite Life I'm mainly watching it for the character of London Tipton, and the obscure friendship of London and Maddie. The Tribe because it's an addicting series with interesting characters that I must get the DVDs to since it's already taken over my brain, and Battlestar Galactica because it's all about the Cylons, dude. Yay for Six and Boomer. I also heard about the mindfucking episode, "Downloaded", from the second season.