The crystals. If you're on a 1.4.7 pack, just make yourself a quick x-ray system (TNT/glowstone through the head) and you'll see why. There's also how they damage everything they touch, so tough luck if you mined a diamond that just happened to be right above one of them!
At least it looks like both those things will be fixed in the 1.6 release.
At least it looks like both those things will be fixed in the 1.6 release.