I came in on the very tail end of 2nd edition, so my memory of it is already gone lol. So as a newish player, I have a fresh slate to work with.
3rd edition seems in whole a good one to me. The Battletactics I really enjoy, however if we are to have global tactics AND book tactics, GW needs to ensure the book tactics don't have any 'auto-achieve' factors in them. We could even trim the book tactics down a bit at that. I personally enjoy the Nighthaunt, "all ghosts, no brakes" (better known as one stop, no return) with the Black Coach being the only model to complete it.
Terrain, specifically mystic terrain features- I've encountered several players who just roll a dice per terrain. I mean, they point at a terrain, roll a die. Point at the next terrain, roll another die, rinse and repeat. I really don't believe this is what was intended, so if my opponent and I agree to play with the terrain rules, whomever is defender rolls all the dice, and divvies it out to each terrain as they like, then we pick sides. From what I've seen and experienced, it's been "where's the arcane terrain" or if you are certain armies "where's mystical?"
They're boring and not fun, imo. It's honestly easier to remember wyldwood, cover, and impassable versus a pool of 6 rules that either hurt me, or do absolutely nothing for me.
List-building - The battalions are rough. Maybe its because I'm still green, but playing Nighthaunt with my little pool of wizards, it really puts me in a hard spot. Do I want more drops to get knowledge of my opponents' deployment, or do I want less drops so I can dictate who goes first? It also ties in with deploying, because I want to keep my wizards safe and away from the enemy wizards, but within the nighthaunt 'bubble' to get my spells off.
Keep double-turn. I sometimes enjoy having a short game just to get to lunch early or buy a beer with my new friend who was my opponent. XD