Agent's '3rd person serious sam' is apt. With an RTS, you are basically a god or commander controlling an entire army. With an RPG, you play a single character and you build up that character with increasing experience, inventory (shields, weapons, armor, etc), and depending on the class, spells and magic. When playing multiplayer, you and a bunch of your friends share a single-player story in co-op mode. In pure single-player mode, it's just you.
Diablo randomly generates the maps and the creatures and badguys actually increase in number as more people join in the multiplayer game. The people that are playing in coop will all level up and gain experience together as they kill badguys or beat bosses. But the randomly generated maps just mean that a church will have a different layout and may be located in a different location if you start over and play again... but the church would still exist... somewhere.
One of the strengths are the cut scenes, the story line, and voice acting, and the general fun of playing a necromancer and raising the dead.

Weaknesses are, for me, Diablo II caused you to wear out a mouse really quickly with the sheer amount of clicking you have to do. From the gameplay trailer for Diablo III, it looks like they might be fixing that issue where there are more unique attacks available.