First is the novelty factor. We've seen these guys time travel before so the whole "explaining time" aspect isn't all that interesting. Second, there's no consistency. The first movie had a plot - it was an adventure from start to finish and you were with these characters the whole way. It was straightforward and it worked. Here, they try to save Lou from being killed by going back in time to stop the killer, but instead they go forward in time and apparently the killer is from that time and ended up going back in time to kill Lou, or something. It's just a dumb convoluted plot. Not that plot is important in a comedy; it just helps to have some sort of narrative to follow instead of scene after scene of ridiculousness.
And third and most importantly, Hot Tub Time Machine 2 is missing John Cusack. He held the first movie together like glue. He played Adam, a likable everyman that the audience could relate to, and it helped that he had some of the funniest lines in the movie. This movie is all over the place without him. Lou is now the main character, and as much as I love Rob Corddry, his character gets really old really fast. His entire character revolves around gross-out humor and shock value, and it worked wonders in the first HTTM because Adam's sense of reason helped balance it out. Jacob is now the only voice of reason but he's Lou's son and serves as the punch-line to a lot of jokes in the movie. Sometimes it works, sometimes it doesn't.
Adam Scott is the newcomer playing Adam Jr., the son of John Cusack's character. The gang meets Adam Jr. in the future and he ends up going on the adventure to find Lou's killer with them. I love Adam Scott and he was fine in this movie, but he didn't really add anything noteworthy to the group's dynamic. He plays a geeky character who's about to get married, then on their adventure as you can imagine, drugs get consumed and mistakes are made. He definitely provides some laughs, and the group is still enjoyable to watch for the most part, but you can definitely feel a void in the character department.
The most important thing about a comedy is obviously the laughs, and this movie has a few. Much of the dialogue feels like shock value for the sake of being crude. Again, it works sometimes. Other times, not so much. Everything was done better in the first film. This definitely feels like a cash grab movie, especially since they didn't even bother asking John Cusack to return because they knew they'd have to pay him a sh*tload of money (although he does have a nice little cameo in the unrated version). But all in all, it's not terrible. It's a run-of-the-mill raunchy comedy with some funny moments and a lot of disgusting ones. It's worth maybe a one-time watch - if you were a fan of the first one you'll find some good chuckles here and there, and if you haven't seen the first one then you might enjoy this even more because you have nothing to compare it to.
As far as sequels go, it could have been a lot worse. The fact remains though that Hot Tub Time Machine 2 was simply unnecessary.