The 501(c)(3) is a bit of a pain to set up (filing paper work and getting approval), but running it is not so bad. They require a board of directors, quarterly meetings, and I believe there's a new requirement for annual filing with the IRS.
At least in the U.S., I'm attending seminars on new business creation, and even if we set up an LLC (the simplest, lowest cost format) for our group, you have to pay a state tax that varies, but it's about $600 a year California.
I guess the main question is, "What are we trying to achieve by incorporating?" The only concern I have is if an individual owns the domain, and then something happens to that individual, will the community live on (i.e. will another admin have rights to pay the bills and keep things going?). Just my two cents...