Believe everything you've read: this game is amazing!
The boss fights are the biggest improvement from AA. In AA, I felt like there was more and more dodge-smash into wall-attack, with the occasional pseudo-boss fight to mix things up (was Harley a boss? Scarecrow? Killer Croc? Not really.) AC made boss fights fun, with one in particular being incredibly creative.
The sandbox was done really well. It could have been a pain crossing the huge city, but it really wasn't. You can get from one end of the city to the other in just a couple minutes; but that doesn't mean it's small. The gliding-grappling is really fast and fun.
Also in the sandbox category, the amount of side missions is awesome. While other games throw you out of the action with a "would you like to purchase additional content?" popup, AC comes packed with dozens of hours of extra fun right out of the box. I'll be gliding towards a main mission, stop to collect a Riddler statue, get distracted by a fight, then by a side mission, then decide to beat up some goonies...it's delightful. There is something to do anywhere.
With respect to this detail: It amazes me how fast Rocksteady pumped this out. Both AA and AC came out between GTA4 and GTA5, and people rave about GTA's depth. "They spend so long to develop such detailed cities" I've heard. Well AC did it in a third the time.
I could go on and on. But you should stop now and a)read a professional review and b)buy the game.