The percentage of weapon damage is probably the best mechanic they did. If Skyrim balanced magic and shouts using that it'd be very interesting. (even if using spells in both hands you'd have an "equipped" staff or weapon that gives base damage) It'd solve a lot of scaling issues in that game.
I also like how the rune system can drastically change abilities and the overall idea of having fewer but more flexible skills.
Unfortunately when it comes to things like art direction, plot, and enemies it's the same generic Blizzard game we've had since Warcraft III. I've already complained about some of the character design, and the point still stands.
It's odd in that the game mechanics, combat, etc. I like but the overall art direction and setting is such a rehash that I can't find myself to want to play after going through it once.
The odd choices on randomization also don't help, with overland areas having "chunks" randomized but not to the full extent to Diablo 2. There's also some set pieces that they don't randomize that probably should have been
the areas leading up to Leoric for instance should probably be more randomized
Also by not really including bosses/quest objectives in sectioned off non-randomized chunks of the map you lose all of the fun that made certain things like the Butcher work in Diablo. The new event setup is nice, but it feels like they're few and far between when every level should probably have 1-3 events.