6th edition has some issues, but I think they all stem from some simple core rules problems. Mainly flyers and wound allocation. I don't think their approach was all bad, they just failed in execution. They didn't anticipate the full effect these changes had across the system.
We all hoped flyers would be balanced out in the codexes. Everyone would get their own jet planes and AA units, and it would all work out in the end. So far this has been inconsistent. Most of the new flyers have been underwhelming. Plus, outside of Tau no one got decent AA options. As a result there's not much to challenge the trifecta of Nightsythes, Vendettas, and Helldrakes. Well except for Wave Serpents, but I'll get to that later.
#1: Change snap shot rules to -2 BS instead of always BS 1.
For a good chunk of the field, things stay the same but it has a few effects. You still have an advantage for the 4, 5, and 6 BS models you pay the points for. The small elite armies like Marines need an answer to Helldrakes. Letting some of their units act as supplemental AA that can hit on 5+ instead of 6+ would help in that regard. It also will forces flyers to stay away from elite ground units because their combined small arms fire gets scary. And what's more cinematic than your BS 6 warlord pulling a Patton on a flyer bearing down on him, hitting on 3+ with his plasma pistol?
#2: Weapons that fire in overwatch can't fire in their next shooting phase.
This change goes in hand with the previous one. If we make snap shots better we have to compensate for their increased efficiency in defensive fire. Makes it consistent with interceptor fire in that all out of phase shooting forfeits your next shooting. We also clarify that shooting phase rules like Multi-trackers don't work in defensive fire.
#3: Allow focus fire on armor saves like with cover saves
Wound allocation is a major time sink and NPE in this edition. In concept it's cool, but it's exploitable. If I can fire on the guys outside of the woods instead of the ones inside the woods, why shouldn't I be able to shoot my lasguns at the cultists I can actually hurt instead of the Chaos Lord in impenetrable terminator armor. A slightly altered way to word it is to say that the wounds must be taken on models that have the majority armor value first. Though I think the first way is better from a pure rules simplicity standpoint.
#4: Something has to be done about that Serpent shield
It's ridiculous. Even as a life long Eldar player it shames me. I'm torn about the actual fix. I think the range is too long on it for starters. My gut says to strait up reduce the range to 18". A fairer option may be to just attenuate the number of shots as the range increases. So you get D3+3 at 12", D3+1 at 24", and D3 beyond that.
#5: And that stupid Hellturkey...
It's too versatile. Either it should loose torrent, or template weapons should be disallowed while a flyer is zooming. Make the thing enter hover-mode to use the baleflamer.
#6: No joining MC's with IC's: Period.
The Riptide loophole is ridiculous. Close it.
There's probably some other balance issues that should be addressed, but I don't want to get to nit-picky. I don't want perfection, just playable. Good enough is good enough. I can live with random charges and things like that. Randomness to a certain extent can be mitigated by play style and good list construction. The goal here was to get the major items that are standing in the way of this being a fun game. The really obvious missteps that a design team that was paying event the slightest attention would have FAQ'ed into oblivion months ago.