Forum Moderators: open
You either have the scenario of creating multiple sites where you'll run the risk of penalty because too much of the content is identical. Most of the time a bunch of sites like that also tend to be cross linked which could lead to disaster in the end.
Or, stick to one site. Drawback here is that if your "categories" ar far too different from each other, then maybe you might have problems making search engines understand what on Earth your site is actually about. :)
I strongly advice you to spend a considerable amount of time here at WebmasterWorld searching for earlier threads about this subject. There are many good arguments both for and against the creation of multiple sites. Cheers!