After having played through Orcs Must Die! 2, I echo my first impressions: more of the same with some improvements. If you enjoyed the original Orcs Must Die!, you will like Orcs Must Die! 2 more. It is good action defense.
Difficulty is moderate. Returning players will beat most maps on the first try but not with a five-skull score. You are not playing for the story, so you unlock the next level to have access to the new map, not to see what happens next. You can always dial the difficulty down; once you have beat all the maps, you can dial it up.
As I said, there is a grind for “character advancement”: upgrading traps, weapons, and trinkets. The grind is short, past which you are just adding options. You can respec costlessly at any time, so you only need enough skulls to top off 10 things, and you only need 10 fully advanced if you are competing for high scores. I am in the 250-skull range, and I capped my favorite traps while getting a small array of options without needing to respec between them. (Easy grind: a “perfect” Nightmare level nets at least 9 skulls for every repetition, and there are a few quick and easy Nightmare levels.)
I like the Sorceress. Her base weapon comes with a charm spell, and nothing quite slows the multi-ogre rush like making them hit each other. It is perhaps too easy to clear the game with the tricks you learned in the first game rather than using the new toys. I like the trinkets: convert that trap slot you don’t need into a passive bonus and occasional active ability like “make more money” or “reset all traps” (or hit both at once). The addition of endless mode gives you a way to stay on your favorite map longer and keep building more traps.
Co-op is the great new feature I have yet to try. My friends list is not full of OMD2 players. You definitely can beat the game solo, even when a map has eight entrances to watch, but several of the maps would be much more reasonable with a second wand or shotgun. Then again, several of the classic maps would be much more reasonable with a second wand or shotgun.