I just transferred my old 401K (~$5000) to my current employer's plan for several reasons:
1.) my former plan didn't have an index fund
2.) my former company was somewhat shady and I just didn't trust their business practices
3.) my former company sent me the quarterly statements but never in a timely fashion
4.) my former company's benefits coordinator position turned over on average 2x a year!
5.) I wanted all my 401K money in one place in order to simplify my life a little

Obviously, this is a really personal decision, but I'm incredibly happy with my choice and am sleeping better at night because of it.

Keep in mind, I believe that if you rollover a pretax retirement fund like a 401K into an IRA, you won't be able to add any post-tax contributions to it. I think they call that "comingling" funds and the tax consequences can get pretty messy. At least that is what one financial advisor told me once. It might be bogus - anyone else know?