The AV Club reviews of the series basically nailed my problem with the show: The writers fell in love with their own character. Dexter was no longer a fucked up monster carrying out unspeakable acts, he was a wounded little hero who needed protecting, and all of his actions were understandable and justified because the world just didn't love him enough. Every opportunity to build up tension and potentially have consequences on the character unfailingly fizzled and it just became boring.
Add to that the constant reset button. In a show like Breaking Bad - hell, even Sons of Anarchy, which isn't even that good - the writers don't shy away from messing up the status quo every season.
It's not that the reset button is a bad thing, because it can work. In a show like Justified, it makes more sense since it shows how stuck in a loop both Raylan and Boyd are until one of them shoot each other, but in Dexter it meant the writers doing away with elements of the show that were too hard to write around.