De-incentivizing play was one of the central concepts I formed before writing my first game. Back in the late eighties, I wrote a version of what later became my first released game (StarCluster) back in 2002. I call it StarCluster Zero Edition, and it is now in its third edition. It was painfully typed out on a typewriter, and I still have it somewhere. I was already sick of experience points, and the way they pushed play - "I'm five XP from leveling up! Is there a rat in the alley? I want to kill something." - that was an actual quote from a game session. This didn't model anything I wanted to do in the game, so first I discarded them from my AD&D game, then still running, by switching to a model of per session increases. The group had no problem and continued on their merry way - with some beneficial changes explained further on.
When I wrote the StarCluster Zero a bit later on, I avoided all trace of XP, leveling, and
the like. Instead, the character's age determined his skills. It didn't
matter whether that age was gained in-play or out of play - you could create a character of any arbitrary age, then advance her during a game campaign, then gen them up a couple years in-between and be older, and there was no functional difference between that character and a character who had been in play the whole time. No "reward" for inching the bastard up through some bogus "Hero's Journey" from incompetent to functional to ultra-competent.
Balancing that, I dropped the concept of inflating Hit Points as well, creating a derived stat from combining your physical stats and using that as an ablative pool. Balancing things, I made physical stats deteriorate with the years from the end of that plateau after maturity. So increased skill was balanced with decreased natural ability. I also made the age-related increases small, so that younger characters could play with older characters, and they wouldn't be totally outclassed.
So, the final product looked like this: Slow but sure advancement in skill if the character survived, along with slow but sure physical deterioration, and a life-pool based on those deteriorating stats. This resulted in something that felt more natural. Younger characters were less skilled, but harder to injure or kill. They were unskilled compared to oldsters, but were better at picking up unfamiliar skills. Older characters were very good at doing things they knew how to do, but grew increasingly fragile. That looked pretty organic, self-balancing, and facilitated a somewhat more survival oriented feel.
So, without the incentives, why play? I had already proven to myself that such play was just as much fun when I dropped XP from D&D - that experiment worked like a charm. Besides, Classic Traveller had already shown you didn't need leveling - hell, it didn't really have any advancement at all at first! It seems play itself is a hell of a lot of fun, even without the "kick" of leveling, and very different to boot. Players didn't do things just for the sake of grinding out a few more XPs. They did them because they were interesting, or for in-game rewards, or for achieving personal goals. All the XPs had been doing was getting in the way of that.
With XP, characters had been doing things that gave rewards preferentially, whether or not those things in themselves were interesting to the player. Once doing things for rewards had been stripped out, players began doing things they wanted to do. It fostered a revolution in tactics - if you don't get XP for killing things and taking their stuff, it's just as good to scare them away or trick them. The mission is accomplished either way, and it makes characters built on things other than combat far more viable.