The garbage in / garbage out thing has already been mentioned, but I thought I'd get a little more specific on the garbage points that bug me.
1. Performance quality: While you can pitch and time shift a performance to be on key and in time, you can't edit in emotion/feel. If the level of musicianship is high enough, who is going to think about the quality of the recording?
2. Arrangement: Does the song have an ebb and flow to it? Are there changes in texture, and do they make sense? You can fake this a little bit thru mixing and editing, but if it's not in the song, then you've got your work cut out for you.
3. Instrument quality: I don't have anything against affordable gear, but gear that's not properly maintained is painful to record. If your guitar isn't properly intonated, microphonic pickups are causing extra clanking sounds in your bass, you've got grounding issues that's causing things to buzz, or your drum hardware is squeaky or not stable enough to stay in one place, you're building barriers between yourself and a decent recording.
4. Bass mud: High pass your stuff. Just because your guitar tracks, vocals, and snare sound awesome and full when soloed without the rest of the mix, doesn't mean they'll fit in with each other. This is especially true in the bass frequencies when dealing with close-miced stuff.