Personally, I feel this is an awesome game. Does it have that new car smell? No. But what it does, it does well. It suffers from classic Part 3 syndrome. When a game hits the third iteration on the same console it will almost always start to feel samey. But, the fighting is still as good as ever, the graphics amazing, the sound very good and the story awesome. There is a lot more put into the detective work for, you know, "The World Greatest Detective". Sure it is fairly scripted detective work, but it has always been that way - you just feel this time like he is actually figuring things out.
The boss battles have gotten so much better this time, they were limited in game 1, interesting in game 2 and even better in this one. People complain about the Deathstroke fight, but it isn't that bad if you learn to use Parry as a reaction instead of button spam, you actually learn to become better at a skill. After that fight I'm able to take on much more challenging groups of thugs. Deathstroke really feels like he should, He uses a variety of weapons and doesn't pause for long, drawn out monologues - he goes right for the jugular, and uses a variety of tools to do it. And the initial encounter with the Electrocutioner is classic Batman. (Not spoilers, the game tells you about those guys like right away)
The biggest complaints I've seen that are valid is lack of a civilian population, which is explained via a curfew on the city. And secondly is a lack of an excessive amount of "Easter Eggs" which most people never saw anyways without looking online. It does have a bunch though, and IGN specifically called it out for that - yet has an entire section dedicated to it in their Wiki (and it isn't a small section, which tells me they were just harder to find).
In the end, if, for example, you are a Gears of War fan and thought part 3 (not going into 4, because I think we all felt that by the fourth game) was nothing but a level pack, then this one will too to you. How important is breaking new ground to you, and did you get done with Arkham City and want more (Harley's Revenge doesn't count as "more")? It's refined, however the enemies aren't as mainline if you aren't a comics fan, but they are interesting, and the story is really nice. It isn't groundbreaking, but third games usually aren't.