So this is a subject I've had bouncing about in my noggin' for some time.
I think that the 5th edition ruleset is very tight. There might be a few things I would tweak here and there, but it's a solid foundation upon which to build a game. Most of the real "issues" seem to stem from codex rules, and not the BRB themselves.
There are, however, many who really and truly hate this edition. I'm sure you've heard a rant or two from these naysayers in your time. The "usual suspects" are Wound Allocation and True Line of Sight. Though I've heard some expressing frustration on other topics, those two seem to be the big ones.