Jerrod Balzer, mackeral eater, writer, and editor at Skullvines Press, has a pretty good piece on why self publishing is bad. Here’s an excerpt:

I also learned why self-publishing is such a bad deal.  When you go this route with your work, you eliminate the opportunity to go through the proper channels.  You need those rejections from submitting over and over.  You need the constructive criticism to help grow as a writer.  You need to learn through many rewrites until your work is accepted.  As perfect as I’d thought OAK CLAN was back then, I look at it now and see all kinds of issues.

Furthermore, once a book has been self-published, no legitimate publisher (as in one that pays you), will take your work seriously.  It raises too many red flags. The work is automatically lumped together with all the countless unedited pieces of crap that are out there.  It will be assumed that you, like others, just stuck your first draft out there in book form.