It's hard to put your finger on it. But I've always been a fan of the heavyweights so naturally I'm going to be biased towards the heavier weight classes. Some of them are just too small that I can't get into it. There are also no compelling personalities and no fights that sound desirable besides Aldo/Edgar. I'm not the only one here, as anything below WW has failed to draw pretty much in UFC history outside of BJ Penn.
I don't think a casual fan would post on an MMA message board but I'm not a hardcore MMA fan so I guess I'm somewhere in between?