Sound-wise I think I'm pretty "forgiving", but if I do notice an actual flaw, that needs to be fixed one way or another. Such as the channel balance being off mentioned in the first post... I've actually been having that problem sometimes, but I never could nail down why it happens. Always goes away after while even in the rare instance when it does happen, though. But still annoying.
In terms of sound, one big no-no would be too much sharp sibilance or such in the treble. Had a DVD player with that issue back in the day (well, it's still probably in storage somewhere). Presumably a very crappy DAC in it to cause such a thing. Very fatiguing and annoying.