I think the biggest flaw of the movie was how unintimidating the Predators themselves were. This is to be expected compared to the first, because of how familiar the audience is with them now.
Still, I think they dropped the ball there a bit. They kept them off screen and built them up for the first half of the movie, and the "Jaws effect" was working nicely. Then they crash onto the screen in a blaze of glory. This was good too...
The problem from here was mainly that they were really downplayed after this. They didn't do much more in the way of being terrifying enemies.
They didn't really get "clean kills" on any of the characters. Two characters killed Predators on their way out, and even the comic relief convict character (from The Shield) got offense in before dying. It sort of lessened the Predators.
The penalty of this is that when our main character gets into a brawl with the Predator, it's not really anything special that he's capable of putting up a fight. The final act of the movie had a lot of shit going on and felt a bit anticlimactic. But I disgress. It's only a Predator movie.
The good things are that it felt like a spiritual successor to the original, and had a great concept. There were some great moments and action sequences. The tone of the film definitely harkened back to the original, and there were nice touches like the music.
The Laurence Fishburne character was good stuff.
The female character recalling a briefing she recieved about the events of the first movie, (and sharing all kinds of details including what year it occured and how Arnie knew how to kill them) was a bit of a facepalm moment. It was pointless, since it didn't have any effect on the plot and only served to reference the first movie in a far-fetched connection.