I just finished reading an article that talks about the hallmarks of an amateur mixing job. Bobby Owsinski lists seven things that a lot of beginning or amateur mix engineers commonly do (or don't do) in their mixes that prevents them from sounding truly professional.
I agree with most of the things on his list except maybe the last one, which he calls "Dull and Uninteresting Sounds." His main beef here is the use of sounds that are either dated, or already used by a lot of other recording artists, like generic Roland synth sounds, the extreme Auto-Tune effect a la Cher and T-Pain. I don't really think those are mixing issues. You can have a perfectly professional sounding mix using dated and/or common sounds. Obviously T-Pain and Cher had professional mixes using those sounds.
One additional mixing mistake I would add to the list of amateur mixing issues is the lack of proper use of the stereo field. I find that folks who make amateur mixes tend to put everything either too much in the center of of the sound stage, or too far apart, leaving sonic holes or bunching things up too much. I know that my first big "aha" moment was the discovery of all things stereo - what it was, and more importantly, what it wasn't (panning two versions of the same audio, for instance).
Take a look at Bobby's list and see if you aren't making some or all of those same mistakes. Check it out here: http://www.prosoundweb.com/article/defining_characteristics_of_great_vs_amateur_mixes/