A few notes about things in various RPGs that I like.
I really like a degenerative armor system. That is, when you take a hit under certain situations, your armor becomes less effective.
Skill based systems.
I like a fate point/benny/karma thing in my games.
I like advantages/disadvantages (or whatever you like to call them).
A dwindling mental resource mechanic (when necessary) is fun, IE Call of Cthulhu Sanity, Vampires Humanity, or Cyberpunks empathy/humanity mechanic. This is a mechanic which has to be used carefully, in some games the loss is too easy and recovery too hard.
Abstract monetary systems
Equipment/gear as advantages
Palladium’s hand to hand combat system