No OA's and advantage is completely up to the DM. Combat felt like a game of Dodgeball with a lot of running up, hitting, and then backing off.
The rogue felt useless, until the DM ruled that any flanking granted advantage and then the Rogue player started having a lot more fun.
The wizard felt that spamming shocking grasp was by far the best tactic. You could run up, get advantage against any opponent with metal in their armor (per the spell), and then run back behind the fighter and the cleric of moradin. The other spells all felt a bit underpowered.
Many of them felt like they should be castable as a reaction or free action. Shield felt like it should be a reaction, and Death Ward felt like it should be free too. In almost every case, doing damage was considered the most important thing you could do on your turn, and doing anything else felt like a waste of a turn.
The first thing any intelligent enemies are going to do is splat the wizard. And there is nothing to prevent them.
I like the lack of AoOs/OAs thus far. I remember when 3e came out, there were so many feats that had to do with AoOs and I thought "My God, they're really forcing us to play with a grid for any of this to make sense!" For a group that had never used minis at all, this was a big change and one of my key objections to 3e at the time. Of course I'm used to it now, but back then it seemed very invasive. Anyway, I think the removal of OAs is one of the key elements of a combat system that will work without a grid. OAs are dependent on very precise placement of creatures.
The same could be said about flanking, by the way.
Did you like it this way or find the movement without repercussion style a downside?
I like AoO's and like to see them included. In Pathfinder Beginner Box where AoO's are not present I ran into situations similar to the ones you described. It was not a good thing in my opinion.
Subjective opinion on my part of course, just curious if you liked this style of play or did you think there should be something to help restrain movement without repercussion a bit?
So, does that mean, that in these cases
* Character turns around and flees without further defending himself?
* Character is attacked by unseen enemy (suprised)
* Character is flanked
the attacker simply has advantage?
And so like I said before, it sort of turned into a game of D&D Dodge Ball. The melee guys set up a front line, then the rogue and wizard would dart forward past them and attack and then dart back. The monsters kind of did the same. They would rush up, attack and then rush back.
In 3e it was kind of the opposite. AoO's were so punishing, that combat became very static. You moved in, stood toe to toe and traded blows. I never liked that either.
Simply? Advantage/Disadvantage is a HUGE deal. It doubles the chance of the result sought.