I have played and DM'ed a lot without A.

's and in general it makes the game faster and adds a lot more movement to the game and in some cases with the right people is better,with the wrong people it blows chunks.
Attacks of opportunity do not make a lot of sense in general and can be stupid and game breaking as well.
You have 6 seconds of time. You take your turn and move and attack and defend ect...but afterwards you can do more stuff in that same 6 seconds? How? time travel? It's just as silly as no facing rules. My fighter is surrounded by goblins but I do not have my back to any of them...yeah ok.
D&D combat doesn't reflect real combat in any way and certainly doesn't reflect fictional combat from any of out books or movies at all either. 95% of the time its stand there in one spot till you or they die.
It's very difficult to actually run away (unless your like a rogue) and its just kinda boring.
Also attacks of opportunity do very little to protect anyone. Maybe at low levels I guess? but anything past the first few levels you get 1 A.

per round ...lol ok you go ahead and take it..ouch you hit now im gonna go pound the crap out of the wizard.....nope still the best thing the party can do is keep the channels to the wizard blocked and fight at choke points......the same as we did back in B/X(but we used a free attack if leaving a fight then to,,,but don't tell anyone,maybe it was a house rule)
Now I sound all opinionated and anti-A.

. but im really not. I used em most of the time and shrug no big issues. it seems like a bigger deal than it is......BUT
Mostly because I don't play D&D like a war game. The goblins charge the guy in the front mostly because he is ...in the front and menacing them and because they way outnumber the party and think they are going to stomp all over the party. I mean if either side played like a navy seal team the entire party would ignore the Lich's minions and simply single target the Lich right? well whats good for the goose......the lich and all his minions should single target the party wizard and then the cleric and then rogue and blah blah blah....but its a game......and we are all here to have fun so use A.

. or dont use em it doesn't make much difference.
I have friends that swear it by them and friends who cant stand them....shrug....either way though it's the same fictional reasons to engage or not engage.
Well unless your playing a rpg that lets you go to town and make tons of them .....in that case they really do effect game play more and in both good and horrible ways so...........lol I guess your still in the same boat but it matters more.