In Diablo 2 the skill use is basically going around with one of your arms tied behind your back. Sure you might get the job done but it probably won’t be pretty.
Mouse 1 and Mouse 2
Hack ‘n Slash
Each character type has a certain set of skills to choose from. Each of those skills can be bound to hotkey as the user sees fit. Each of those skills may only be used by mouse 1 or mouse 2, which is also bound to character movement. Once the skill is chosen for the mouse spam to your hearts content until everything in sight is dead, grab loot and repeat.
Skills should be easily switched by using the hotkeys on the fly.
A character can make use of many of the skills instead of relying on just one.
The idea is sound enough but in the truth of it all this set up rather limits variance in skill use as it is not practical during gameplay to switch skills constantly. Beyond this while having skills be the same as the movement function just depending if you target a monster or not you can sometimes get a character to run around in circles trying to click on things with a tiny collision area, damn leapers. Still a very mindless but somehow enjoyable game that the next generation will hopefully fix with a hotkey bar.