The biggest problem is purely conceptual. Part of the joy of LEGO Star Wars was that the game's levels were all based on the films, same with LEGO Indy, and presenting familiar material in a playful format is enchanting. While Batman characters may be familiar, the levels and plot are new ones.
It's a small thing, but it means the satirical flourishes are virtually absent and what there is, is blunted. The comedy rarely raises higher than a character falling on their face. Thankfully, the series hasn't done that.
+ LEGO Star Wars... with Batman
+ Twice the size of LEGO Star Wars
+ Lots of Bat mechanics
- Not really as funny
- And not really anything new