Justice League: War is a direct-to-video animated feature starring a variety of superheroes from the DC Comics universe, and as such it's a bit of unfamiliar territory for me. As I've explained before, I'm no expert on comic books, but I like science fiction and I'm interested in many of the various themes they often delve into so I often find myself watching superhero movies. So as a Certified Non-Expert, I can tell you this: even if you're unfamiliar with the comics on which it's based, Justice League: War is fun and easy enough to watch, but I get the feeling that it's built for an audience knows a bit of what's going on.
None of it is bad, it's just a little shallow in terms of the characters and the thematic ideas. We're introduced too quickly to all the superheroes so that the only one that ends up with any real arc is Cyborg. Part of what made The Avengers great was that it managed to juggle a large collection of heroes with drastically different characteristics so flawlessly, and that's sorely missing here.
But more to the point, that won't be a problem for anyone who's read the source material, and in technical terms the movie is rather well made. The animation looks quite lovely and the voice acting is all pretty solid (featuring Sean Astin, Michelle Monaghan and Alan Tudyk). At the end of the day, Justice League: War is harmless light entertainment with a solid target audience in mind, so if you think you might be interesting in it then definitely don't be afraid to check it out. Just don't expect the level of depth or accessibility of The Avengers.