First of all, the movie is way too long. Parts of it are really boring.
Then, the story is not too interesting. Old story: bad man kills father, son escapes, son seeks revenge, son gets revenge. Most of the details that try to make the story interesting, actually make no sense.
First, at the start of the movie, the fight between the 2 gangs seems to respect some sacred, honorable rules, as if not fought by gangs, by thugs and murderers, but by noble samurai. In the first 2 minutes of the fight, there is absolutely no blood on the ground, on the clothes, faces or weapons of the fighters although all of them are wielding knives, axes, swords and so on. Suddenly there is some blood on the actors, but on the snow there is just a little pink paint. It looks really stupid because a fight in which about 200-300 men take part, and they all have knives and axes and swords, there would be dozens of dead and the place would look like a butchery. Also, the police could not ignore something like this.
Then Day Lewis's character looks to be though out as a love-him hate-him kind of guy. He is a monster, but a monster who loves his country (Hollywood BS) and who respected the man he killed in battle (DiCaprio's father, Liam Neeson). Although Day-Lewis makes the most of this character, the character itself looks surreal: a gang leader/gentleman/racist/patriot/sadistic/brave/illiterate/intelligent butcher. This is actually what this movie is about. The story fades behind Day-Lewis's character.
There are some other characters that seem to be in the movie just for the sake of it. The politicians, the wealthy family that at some point visits The Points - the square where most of the action takes place. These characters have no role to play in the story, yet they show up from time to time as if only to have something to fill the 2 and a half hours of the movie.
The other thing that has no place in this movie is the riot. The riot and the main story have no connection. The riot does not influence the outcome of the story nor vice-versa.
The movie is barely watchable, and if the weather outside is fine, I'd suggest you rather take a walk in a park.