Probably the batmobile. It sucks.
I know most don't like it. But I think the addition of the Batmobile is awesome. I'm only at about 22% completion but so far I like the ride a lot. It adds a bit of immersion to being the Bat. I know that I can get around the City faster while gliding around rooftops. But right now I prefer driving because its fun and the Batmobile looks and sounds so awesome.
This game is done so well. The graphics are spectacular. And I really like how the developers included some of Bat's other villains. The combat is really refined now and environmental takedowns are really cool to watch. And they seem to work on the more tougher enemies which is a great way of eliminating the greatest threat quickly. So much stuff has opened up that it feels like there is too much to do. I don't know if I'll complete everything but I am having a really good time with this title.