PvP is a trivial way to make a game difficult. It's not easy to make a good PvP game, but if you do, the game will be difficult automatically. But without changing difficulty, you can make a PvP game be less dependent on skill by making it more dependent on gear or luck. See? Double dissociation.
Puzzle games require puzzle-solving skills. It's a skill too.
Street Fighter is an excellent example of skill-based combat. Thanks.
Also, free respecs, which is a huge bonus over the D2 of back when I was playing it.