This is going to be a bit shorter. Trine was a charming game, especially audiovisually. It might have not been the most balanced game though and the utility of its three characters was pretty far from equal.
The biggest problem was the knight who had almost zero utility in the first game. He was designed to do the fighting, but quite often the thief's arrows were able to deal more damage and were safer to use. Since the thief was also able to swing around with her grappling hook, for me she stole most of the spotlight. However, this being a physics-based puzzle-platformer, the supreme character was the wizard, able to not only move objects with telekinesis but also to create new ones out of thin air. Limitations were involved, but even with them the wizard was a puzzle-solving powerhouse.
The level design in Trine however enabled so much utility for the thief's grappling hook that I didn't need the wizard that much either. I guess it was more fun to just swing around although at times it probably took me more time in form of attempts than it would have taken to just create some boxes and let physics do the rest. This is all good though, allowing the player to make choices in solving puzzles. This is generally an advantage of physics-based games that players can always come up with solutions that developers did not think of. The possibility space in Trine certainly is not very large but the illusion is usually good enough.
Like any sequel worth its salt, Trine 2 changes things around a bit. It adds new kinds of puzzle devices into the game mechanics. This creates more variation but also I felt that most of these new solutions made puzzles more "hard-coded" to one solution. The thief's utility has been decreased by reducing the amount of surfaces that can be grappled with the hook. The wizard has also lost his triangle block which was the only object he was able to levitate around while standing on it. The knight still hasn't gained any really new utility tools and the thief is still pretty damn solid in combat.
The balance has indeed shifted but to what direction. I felt that unlike Trine, the sequel actually forced the player to use the wizard's abilities much more. Many of the puzzles were not solvable with other characters' abilities at all. The knight still has only two purposes outside battle: protecting himself with his shield and breaking things with his hammer throw (which might or might not have been new, can't remember) - both very contextual. Thing is, the wizard is the only character in the game whose abilities do not require much support from the level itself. He can always create boxes and planks. With four objects total, a lot of things can be done.
Balance problems always exist in games with more than one character or class to choose from. Of course, it is not too bad for Trine, because characters can be switched instantly and constantly. On the other hand though, being a puzzle game does by puzzle design dictate which abilities are more useful. It can require all abilities but rarely does so in equal amounts. The wizard clearly gets more screen time in Trine 2. Fortunately, his problem solution model has a lot more variance. Creating a stack of boxes is not a solution to everything. In fact, the last two or so levels in Trine 2 had pretty devilish design. This of course comes with a price - the more difficult you want a physics based puzzle to be, the more "hard-coded" it's solution is likely to become.
All in all though, Trine 2 is a somewhat better game than the first one. Both are quite charming but still have their flaws. I still think the knight could use some ability that makes him more useful. Hammer throw spots aside, there were very little places in the game where his abilities had anything to give to the solution. At least he should be even stronger in combat as he is still outdone by the thief in many situations. The biggest improvement in Trine 2 is the addition of new puzzle mechanics such as watering mystic plants to create new paths, and also the increased difficulty in later levels.