Good idea - Rainbow Six Vegas 2's experience system

Non-RPGs have often dabbled with adding in experience systems, and recently the idea seems to have really taken off in first person shooters. Halo 3 had fairly limited multiplayer progression, and then Call of Duty 4 had a fantastic online experience system. Now the Rainbow Six Vegas 2 (and that's the last time I'm ever going to type the ridiculous football score name out in full) developers have implemented a logical progression from that.

In R6V2 you have a single character, who is shared across all three game types - campaign, multiplayer, and terrorist hunt - and every kill that you or your team makes earns you experience (though obviously you earn more for making the kills yourself). At certain experience levels you earn a new rank, and with those come new types of armour and camouflage.

R6V2 also adds the A.C.E.S system (I forget what it stands for), that rewards you for kills made in certain ways. So shooting enemies at long distance will improve your sniper level, and killing filthy terrorists with explosives increases your assault rating. Leveling up these categories gives you experience bonuses (towards your rank)and is also how you unlock weapons, themed to the category you improve.