Tempo and timing. Then again, being a percussionist, I tend to notice those things fairly quickly. If someone is off or going too fast/slow than the rest of the band, I can noticed it. A good band will flow together, have tight timing and generally do not fall apart when someone throws everyone else off.
Even bands like Dream Theater suffer from some bad timing every now and then, but it can't be helped. We're all only human after all.
But for every band that has a bad performance like that, they can have a dozen, a hundred, or a thousand great performances. So many factors can change how a single performance can go, and not be entirely indicative of a band's skill. If the band is just starting out, then it might be glaringly obvious if they're not performing well, but without performance practice, you cannot improve, so having a band show or two (or a dozen) will help the band, as a unit, grow and prosper, to become a tighter group and learn how to work together and off each other.
-Marc.