To me it's kind of a four prong problem that really makes it hard for me to enjoy that subgenre.
1) In a lot of what I've heard there's not much sense of fluidity and that makes the songs really hard to follow.
2) The songs also largely follow the same formula with a clean vocal opening, shouted verses, clean chorus, breakdown, end and while there's nothing inherently wrong with utilizing formulas that work, they do get boring if you're not going to bother using those formulas to experiment with something else. Between the Buried and Me seems to be one of the few bands that falls under this genre tag that actually does this, although I find their stuff falls more into the Mr. Bungle style of experimentation, which is thrown whatever at the wall and see what sticks.
3) The fans. This one is the most personal and interchangeable, and while I'm not going to say all metalcore fans are like this, from my own personal observations at shows and CD stores, they tend to be the most obnoxious and least courteous in the pit. When I was at a Hatebreed show a few years back, I lost track of the amount of time people were purposefully pushed down, which just isn't cool, if you're in a pit and someone falls down, you get them back up, that shouldn't need explaining. Other instances were more of the loud "bro" personality which just made them a joy to be around. Again I just want to reiterate I'm not saying all metalcore fans are like this, but I've seen it enough that I now skip all metalcore shows that roll through town unless one of the opening acts is something that I really want to see.
4) Lastly, it just kind of seems to be this Frankenstein-genre, taking bits and pieces of other genres that are a bit more abrasive and edgy, and making them safe enough to put on a shirt and sell it at the mall. I am aware of how douchey and elitist that sounds and I'm willing to own up to both of those things.