There is a new RPG on the market built my Monte Cook (of DnD 3rd edition fame, among other things) called Numenera. It is a smashup of far future and fantasy, vaguely in line with many Final Fantasy products. The world is Earth but enormously far into the future. Swarms of nanobots roam the world and everything is subject to their machiny whims. Being a spellcaster isn't really about manipulating magic but rather about manipulating tech, tech which is so far advanced from our familiar zone that it might as well be magic. Arthur C. Clarke wins again. I should note that I haven't actually played Numenera because I have too much to do playtesting and writing my own game but I read about some of the mechanics here and I am going to talk about them.
One of the really fascinating things Numenera does is completely rewrite the book on XP. Characters gain XP from doing things and can use it to advance their character's attributes but it also can be used to directly influence outcomes. For example, you can spend XP to alter die rolls to save yourself in combat. The GM is also encouraged to say things like "The enemy has a rocket launcher you didn't know about. You can either take 2 XP and accept this or spend 1 XP and there will be no rocket launcher." Now this certainly is kind of cool and all but it really smashes the fourth wall to flinders. I very much enjoy putting myself into the story and trying to play as though my challenges are preset; essentially pretending that the GM can't just say "You win" or "You all die horribly" at any time.
I feel like having these explicit game challenges where mechanics that fall way outside the character's POV directly influence what happens feels icky to say the least. It feels much like the GM fudging dice rolls to keep the characters alive to me. The suspense just isn't there anymore since the game feels like it is all about the GM being capricious instead of setting fixed challenges that need to be overcome. While I know logically that challenges are designed for me to beat them I really don't like having my sense of immersion in the world blown apart so flagrantly.
The other issue with XP done this way is that there is always a conflict between winning now and winning later. I would feel terrible spending all my XP to win a fight and not getting to advance. I am a lunatic when it comes to these things and when I play games I always have all my consumables still intact when I get to the final boss. Other people are obviously going to have a different take on this but I find the idea of permanently weakening my character to change a die roll exceedingly distasteful. More generally though I am not in favour of mechanics that allow some characters to advance more rapidly than others based on player decisions. I feel like parties are going to be happier when everyone advances together and tactics affect current situations but not future power. Stories may be told with Aragorn and Legolas fighting alongside Pippin and Sam but nobody wants to play Pippin and they don't want the feeling that they might as well not even be there. The best way to avoid that is have everyone advance together.
Given these things I don't think I will try Numenera. If anyone who knows me picks it up I would be interested in reading it though since it probably has some cools ideas I can poach.