Will Arnett (Job from Arrested Development) reprises his role as Lego Batman, the arrogant loner who graced us with the greatest song ever written:
The story is multi-faceted, but largely boils down to Batman needing to learn the importance of teamwork, family, and hatred.
Hatred? Why, yes. One of the driving forces behind the story is that the Joker's relationship with Batman is metaphorically almost sexual in nature, and it is utterly hilarious!
Part of the joy of the writing of this film is how it is aware of the inherent silliness of Batman, while also crafting an interesting and emotionally engrossing story.
The silliness is also enhanced by factors that make me think of it as a pseudo-sequel to the Lego Movie. Those facts are that Gotham is described as being built upon a delicate platform above a void (i.e. a table) and that when characters shoot guns the voice actors make "pew pew" noises - almost as if it's people playing with toys, but it's not explicit.
There is also an odd fascination with Michael Jackson, having both in your face and subtle references. For example, see the choir of children singing "Yeah, Charmone!"
The only real negative point that I can think of is that some of the jokes don't mix well together. A specific example is a point where Batman excitedly asks if he will work with the Suicide Squad, and then around 20 seconds later says that the concept of bad guys being used to fight bad guys (e.g. Suicide Squad) is a moronic idea.
This is a wonderful, exciting and hilarious film. The jokes are relentless, and are mostly brilliant. It's not as good as The Lego Movie, but is still amazing fun.