Ok, so the story is a little predictable but so what? It's still a whole new story with new things to do and new (some of them) people to remove from the streets of Gotham.
The combat system hasn't changed, the gadgets haven't changed (much - the exception being shock gloves) and the overall atmosphere of the game hasn't changed. That, dear readers, is a good thing. Many websites which reviewed this game said that not changing aforementioned features was a bad thing, but I assure you it is not. This coming from gaming sites that consistently rate CoD and BF high, despite them being very similar each time.
The 'Most Wanted' missions are really good and give the game an extra boost, in both time and fun. These can last as long as you want them to because they just stay as markers on the map until you complete them.
There are plenty of reasons to buy this game but to list them all now would take away from the fun of finding them out for yourself! Buy this game!