My greatest complain is the screen shaking in combat. That really, really, spoils the action and makes me stop playing frequently. Games are not movies, they don't need to be a "cinematic" blurry mess full of artificial screen shakes and other cheap effects to conjure up drama!
The issues with the dark gear effects can be fixed with mods. It's one of the first things I do when I play TW2.
I personally have always hated mini games in TW, they're always a mixture of frustrating and waste of time. Dice poker sucks in TW1 & TW2 I don't care if someone likes it. Sitting around waiting for a random dice roll is boring. I'd rather play Pazaak in KoTOR. Arm wrestling at least is quick, but also super easy with the exception of the very last one until you call out the cheater. Fist fighting was way better in TW1 & TW3. I also wouldn't have minded having the drinking games of TW1 brought back (they're not in TW3 either).
Also, once you figure out how to beat them, all of the bosses are quite easy. I can beat the dragon on dark mode in a couple minutes.
I have no framerate issues, the game runs perfectly for me and I wouldn't say that my computer is high end by modern standards (i5-4670K @ 3.4GHz, GTX 770, 8 GB Ram, game is installed on traditional HDD). I'm going through another play through right now. I'm even playing in 3D at times (which looks amazing) and get no lag at all.
Level design I think is a strong suit of TW2, Flotsam is one of my favorite gaming worlds that I've come across. I do wish there was a bit more content (especially since there's a completely unused area past the broken bridge), but the areas are loaded with content even if they are slightly smaller in some aspects. I'd rather a smaller area with tons of content than a huge open area where you just go into an area for a couple minutes and then never see it again.
Combat there are some animations that you always get to work by pressing certain directions and attacking. For example, move left and then left click and Geralt will do the spinning attack that tends to hit the guy like 5 times on the way in. Combat obviously wasn't amazing in TW2, but it was deep enough that you could play the game without doing cheap crap all the time and have a great experience. Obviously TW3's combat is way more refined, but you're talking about going from click click click combat in TW1 to this and then finally a much more refined system in TW3. When I first loaded up TW2 again I was trying to dodge like in TW3 and then remembering that the only thing you can do to get away from enemies is roll. Took me a few minutes to get used to that.
With proper gear enemies die quite easily on dark mode. I have the first set of dark gear and the game is already becoming super easy for me. I also never use quen and only use dodge to retreat from enemies for the most part (rather than rolling behind them). So I don't use cheese tactics too much (except maybe Aard -> Heavy -> Heavy -> Heavy ).
Turn off difficult QTEs in options (poor design to have them in the game anyway I'll admit) and you'll never die in a cut scene. Devs actually expressed the fact that QTEs were a bad choice after the game's release. That's why there's at least a way to disable the ones in cut scenes. The only other time you'll run into them is fist fighting.
UI can be annoying, but it's vastly improved from the game's release. Either way, you can get used to it after a while. Otherwise just get a zero weight mod and you can just hold on to everything and then sort by value to figure out what's the best.
Alchemy was kind of bland in TW2, but mostly because too many potions had tons of negative effects and made you not want to use them. Rook, Swallow, Tawney Owl were the only ones worth using IMO. Then again, in TW1 & TW3 with rare exceptions I pretty much only used those as well. Being forced to carry around liquor to make your potions doesn't make alchemy much better. Also, the alchemy tree in TW2 is the most OP so there's that.