A Different Kind of Experience

Ages ago I used to play a MUD with very interesting ideas. One of those were the way it handled XP: Instead of automatically increasing your total XP, it only added the points to a pool which slowly and steadily increased your actual XP value. Now, if only this would be the difference, it wouldn't worth to mention. But obviously, the system was a bit more complex than that, because the pool had levels associated to the amount of XP in it, and it also had a maximum value. (Which was called "Full of Facts") Naturally, when it was full, you didn't get any more exp, however you tried. Also, the more filled the pool was, the less XP you received for anything. It usually took some time to actually reach FoF, especially on the higher levels, but when you did, you just had to go and rest for a while to empty your pool.
Aside of providing you with the ability of creating a really complex game system around this simple mechanic (ie. talents/skills/item properties to enlarge your pool, change the number of levels, increase the speed of xp absorption, etc) it also opens up the mind to consider other ways to approach experience and levelling.
Here are a few random thoughts about experience - it's not ordered in any way, and I am not aiming to draw a conclusion.

• Experience as collectible item: Let's say the experience you gather is a kind of dust. Based on the foe you kill or the quest you finish, you get a certain amount of coloured dust, like blue or yellow. Slowly you pile up a huge amount of it, and then you can use it to advance your level. But, you can also sell it. Or exchange it with other players for the kind that you need.
• What if we allow the player to pick his/her preferred way to level? For example, what if we say that the player can either choose Classic advancement (XP, as it is now) or Quest advancement (There is no XP, to reach the next level they need to finish x quests - x is growing with the levels) or Exploration advancement (the game don't care about what you kill, or who you help, only what you saw.. think of the mapping feat in the original GW)
• As an alternative of the Sidekick system, what if you introduce artificial barriers in the way of advancement? Let's say you define 5 level ranges (1-10, 11-20, you get the picture) and you say that at the end of each there is a challenge. The players can very simply advance from level 1 to 10, but then they need to spend some time there and grind and quest, and etc, to be able to go on. But, when they finally step to the next level, they have a huge improvement in their power - let it be physical, magical, or anything else. DPS, Health, Defense, all are increased significantly when you enter the next level range, and stays around that until the next range.
  What would this system offer? By forcing the players to stay at a point, we control the population diversity. Simply put, even if your friend is playing somewhat more than you, you can pretty much keep up with him in power.. at least for a while. If you choose to finish the team content at the end of your current range, he will simply wait for you there.
• Spheres for crafting, adventuring, social: Vanguard has this system for separating the experience needed for levelling in your crafting profession, adventuring profession and diplomacy.  This is a great  idea, but I have issues with the practical application. Their approach was to separate these as much as possible, except for a few quests/activities (mainly endgame) where a combined effort was needed. I think this should be the exact opposite. The way I imagine this, it should offer you alternative ways to finish a quest. For example, if the village is threatened by a band of bandits, you can either go there and kill their leader, or make a big bunch of weapons and/or armour for the villagers, or simply go there and persuade them to leave the village in peace. Same quest, totally different experience. Replayability value: High.
• Item XP: There are legendary weapons in Lotro that gather XP, advance levels, get scores to spend on abilities, etc. This is not a bad system, but I have seen a much better implementation for this. In the original Tome rogue like game (Before 3.0) alchemists were able to create artifact weapons. An artifact weapon originally is not much more than a normal weapon that takes away half of your experience, and collects it. The fun starts when you decide to finalize it.
  At that point, you get to a customisation screen where you are offered a list of powers that you can buy. Although you can see the whole list, you are only able to pick the ones that you have learned by studying other artifacts. You also need a material focus for most of them, ie. you need a rabbit foot for to be able to add Luck to the weapon. This sounds like heavy grinding, and in a sense it is - but it is good fun, I really enjoyed it. 
• Now, how about the same with your mount? You buy a foal when you are low level, it collects the XP with you, and after a while you can simply buy speed or flying or running on water from the collected XP?