It's Deadpool 1, only better!
The short of it is, Deadpool 2 takes everything its predecessor made good and capitalizes on it with a slightly bigger budget. The adult humor, the fourth-wall breaks, the present-day in-jokes - it's all here and then some.
Ever since killing Ajax and saving his lady love in the first movie, Wade Wilson has been making a killing a contract killer. But when his bloody rampage goes too far, it takes an overweight mutant teenager by the name of Russell Collins to make Deadpool see that there is more to life than swimming in a pool of dead guys. Cue a reunion with couple of old friends, X-Men, and a very pissed-off Cable and you have the slim workings of a C-movie plot (B-movie at the most).
But it's this poor story which makes Deadpool 2 so great. As I said with the first Deadpool film, the mercenary only works well in this sort of medium. His knowledge that he is in his own movie is integral to the film's personality, so having it be as crappy as can be adds a Deadpool-esque flavor you won't get with higher production values.
What really gets audiences to watch the movie is Deadpool himself, who spouts one-liners and pop culture jokes like the world's worst internet troll. He seems to have more dialogue than all the other characters combined (and seeing as you won't remember the other characters' names anyway, that isn't much of an issue). That said, the rest of the cast does well enough to move the plot along.
Someone who does deserve special mention is Thanos himself: Josh Brolin. His inclusion in two Marvel movies in one year lets audiences see the more pissed-off version of the actor, even though majority of his rage stems from Deadpool. His no-nonsense attitude and downright Batman-like personality compliments Deadpool's more goofy side (just like in the comics).
Since Deadpool's origin story was taken care of in the first film, this sequel has more creative freedom to pursue a lighthearted tone revolving around Deadpool's need to find himself. This makes the film feel a lot more in-tune with its main character than the previous outing (which had the problem of being too overly serious at times). There are story-heavy points where everyone puts on a more actor-worthy face, but these are scarce for the most part as the film puts more emphasis on witty dialogue and action sequences.
And the action here is brutal. If you don't know by now, Deadpool has an R-rating not just for its language, but for its violence as well. You'll see torsos ripped apart, hands flying off of their arm receptacles, and testicles flopping out in the breeze. It can be integral to a scene at times (though mostly it isn't), but if you're one of those people who get easily offended by a severed body part or dangling genitals, do your fellow moviegoers a favor and watch something more socially acceptable. Other people want to actually have a laugh without being judged.
Deadpool 2 is one of those rare instances where a sequel does infinitely better than its predecessor. It's loud, violent, and isn't afraid to blow its load all over your face.