So I finally finished everything there was to do in SP apart from Deathstroke's challenge maps and the "thanks, old friend" achievement because I've got other things to do than talking to Alfred 5 thousand times hoping I'll get a new dialogue instead of him repeating the same 3 things over and over again.
It took me about 52 hours in total to get 100% in NG/NG+, finish IATN and all the challenge maps. I really enjoyed my experience with the game. I replayed AA and AC right before AO, so I've been playing batman games every day/every 2 days for more than a month now so I'm a bit burnt out.
Overall I think I'd put AO on a level similar to AC (that is, better than AA). AO does some things better than AC (the story, bossfights and character design are all way better, and the new enemy types really add to the combat). On the other hand, it's not as polished as the first two games and that can lead to some frustrating situations (snipers always knowing where you are once they've spotted you in outdoor environments is really annoying for example. Also, sometimes -not really often thankfully- Batman's attacks whiff for no reason).
The combat in story mode in AO is harder than in AC/AA, and you need to play the game a bit differently (I found myself using group stun and redirects way more than in the past, since enemies' attacks are so quick). It was a bit difficult to adapt at first but after it was just fine. Challenge maps are easier though, and you can play them in a similar fashion you'd play AC's. Once again, I really like the new enemies, especially the martial artists. You really need to pay attention to what they are doing.
One thing I didn't particularly like though is how late some gadgets/attacks would unlock, especially the sonic batarang and the destroy and disarm move. The game is almost over by the time you get the latter.
After having played the game, I'm actually really puzzled as to why it got scores way lower than the previous games. It doesn't really make sense to me. Were reviewers playing a different game from the one I played?