Acting was certainly good, and the scene with Affleck and Michelle Williams on the sidewalk was maybe the best acted scene of any movie last year.
2 things that bothered me, personally.
One: I am not a big fan of movies that spend a lot of time with showing how "realistic" their characters are. There's multiple scenes worth of Casey Affleck taking out garbage, shoveling snow, driving in traffic, walking on the sidewalk, etc. There's realism in movies, then there is just wasting time.
Two: And more importantly, no one in the movie was likeable. Not one character. I understand it's a movie about the grieving process, but give me SOMEONE to root for. Anyone.
Affleck is a jerk. The kid is a jerk. The kids mom is a jerk. Everybody in town is a jerk. Just a bunch of assholes doing asshole things. I don't care if any of them sufficiently grieved, because I dislike all of them.
I didn't necessarily need a happy ending, just some happiness is the movie somewhere.