1. This movie depicts the bizarre true story of the infamous Boston criminal/FBI informant Whitey Bulger in the late 1970's, and how he manipulated everything and everybody around him to eliminate his mafia rivals and control a lot of Boston's organized crime.
2. This movie is unusual for an organized-crime film because there's not very much action. There are a few violent scenes, but the story revolves around more themes than just "kill my enemies and get more power".
3. The movie is actually about layers of loyalty, and what decisions people make when different layers overlap and conflict with each other.
4. The cast is great and the movie is well-acted, but there are quite a few interweaving stories, so even the great actors (like Benedict Cumberbatch) don't always get a lot of screen time.
5. Johnny Depp is pretty mesmerizing as Whitey Bulger. He almost vibrates with suppressed tension and rage in every scene, and you get the deep, uneasy feeling that no matter how calm Bulger is, whenever someone approaches him, they enter a minefield set by a psychopath.
6. There is a really interesting running commentary in the film about snitching. In Bulger's world, snitching is worthy of death, even if you're snitching on the enemy. And yet Bulger is OK with becoming an FBI informant because it serves his own agenda. So the movie is also about rationalizing your actions, even if technically you're a hypocrite.
7. It took me awhile to figure out why the title of the movie is "Black Mass", but it actually made sense when I thought about it after seeing the movie.
8. The movie is also a very illuminating perspective on corruption, and the old saying "the road to Hell is paved with good intentions." Good people do bad things, and bad people do good things, but these actions can have simultaneously different reasons and results, depending on which way you look at it - and who you're being loyal to at the time.
9. If you want another version of "Goodfellas", this is not the movie you are looking for. And it's not the best crime drama ever made. But it is an intelligent and absorbing reflection on the issues and psychology that underlie the violent gangster news headlines.