Batman is a real badass, the Joker is a total wacko, Harley Quinn is hot. What else do you need? No but seriously, Aussault on Arkham did many things right, the characters are fun (especially Boomerang who is the major comic relief in this movie, the Joker and of course Quinn), the action is pretty awesome and there were some very fine One-Liners also.
What I liked the most was the very mature style of the movie, there is blood spilled, heads are blown of and teeth get punched in but nothing is never totally over the top (as many already said it's very surprising Assault on Arkham was rated PG-13 and not R).
Altogether I liked:
-well animated action/fight scenes
-actuall plot surprises
-real fight consequences (villains DIE, and don't flee with a black eye)
-GREAT voice acting
Now for the things I did not like:
-not enough Joker (yes his role was pretty major but it was not quite enough screen time for my taste)
-overall to short (71 mins? really?)