I think people's ears are better than they give themsleves credit for.
A higher noise floor, a little distortion - I mean stuff you don't really hear in an up front way - just back there lurking around subliminally, are potential culprits. Bad gain structure, cheap cables, little ground loops, old drum heads, bad room sound - they all add up to subtle degradation.
Poorly arranged or poorly written songs are probably another huge factor; less than exceptional musicians as well.
Probably not much about the equipment. Cut any of these forum hosts loose with that gear, some ace musicians and some good tunes, and I'm sure it will dismay you even further.
