Not enough exposition. I don’t necessarily need my hand held (a little info goes a long way), but I’ve read numerous issues where I just plain couldn’t figure out what the heck was going on. And not in an intentional “mystery to be revealed as the story goes along” way, either. The characters will be doing something (that may or may not be drawn from a strange angle or in an odd style, so you don’t even know what you’re looking at) and they’ll be talking to each other in obtuse or coded language, or using short, disconnected sentences, about what they’re doing (or about a different topic altogether, making matters worse). The characters themselves know what they’re doing and talking about, but the reader sure doesn’t. Makes me feel dumb. Comics shouldn’t make me feel dumb.