Equipment Socketing!
Regular Gems(+Strength, +Dexterity, etc.) and gems with unique abilities akin to Diablo-like action-rpgs.

For example, attach a gem that has a chance to trigger chain lightning on hit. Or a gem that has a chance to cause a poison explosion on hit. Or life on hit.

I really disliked having to replace a piece of equipment with a unique ability in D:OS because I found something that dealt more damage or gave better stats.