IE556: I disagree with the definition you gave of a mediocre. To me, a mediocre band is one that's the part of a genre but doesn't really much for it. In other words, it's just one of the many. There can only be a few "leaders," and therefore, there can be many bands who aren't leaders, simply because they came after the leaders, but still kick a lot of ass. If you're not a leader, it doesn't mean you're mediocre (that kinda rhymed!).
For example, IMO, In Flames and Dark Tranquillity are the leaders of their style of music. I would say At the Gates, but since they are no more, they've gone up to the "retired legend" status. Soilwork, one of my favorite bands, would be one of the bands in that vein that's awesome, but still not a leader. Or, IMO, CoB are the leaders of their style. Since our beloved Skyfire are so often compared to CoB, I would say Skyfire is a kick ass band in that genre, but not a leader.
But to fully address the question, mediocre bands, to me, are ones that are just "meh," or "pretty good," but don't really do much for me. They don't suck, but at the same time, I don't really give a fuck (that rhymed for sure!). For my tastes, I'd say a band like Shadows Fall are mediocre: they have some good stuff, but I don't really care about them, and they don't really do much for me to set them apart from others.
This (the example of a mediocre band) was actually pretty hard for me to think of...usually if I find a band mediocre, I don't listen to them, and then just kinda forget about them. So most bands I actually remember are either good, awesome, buttsmacktacular, or they flat out suck.
