The original Trine was like an extremely pretty tech demo with some great, but under utilized ideas. It was fun to play around with, but combat was ridiculously easy, and almost every puzzle could be "broken" by flying accross the level using the Wizard's levitation spell.
Trine 2 is basically Trine 1 with better graphics, less fluff, and superior level design. The puzzles are much more rewarding than in its predecessor (levitation bug is fixed), but combat is still a total joke (still quite fun though, just no challenge). Frozenbyte has thankfully gotten rid of the trinket system, and replaced it with collectible poems and paintings. Aside from that, the game largely plays the same as the first.
I can't help but feel that Trine 2 is a bit of a missed opportunity, being slightly shorter than the first, and not really fixing the easy combat. Despite these criticisms, I still give it a 9/10 because there simply isn't any other game that can quite match the Trine experience, flaws and all. Put simply, if you think you will like Trine 2, you probably will, and the $15 is a low barrier for a great gameplay experience.
(Also there is an expansion coming soon, so there is a good incentive to start playing the original campaign now)