The core elements such as co-op gameplay, Lego blocks, and collectible studs remain, but the rest has been refined, polished and tweaked to provide an even more satisfying and solid experience.
Anyone playing Lego Star Wars with a partner experienced the frustration of not being able to wander more than a few steps away from your playmate before being stopped by an invisible tether. Lego Batman vastly improves the distance two players can separate, making it far easier to play with a pal. The camera pans far back to allow both players to move freely while staying on the same screen; during the demo I saw at GDC, the camera panned so far back that Batman and Robin were scarcely bigger than your thumbnail on screen.
With this and LEGO: Indiana Jones, there's going to be plenty of LEGO cooperative action this year!