Looking through Trollsmyth's pixie class while getting the URL for the link in my brownie post, I realized that my dryad and brownie classes are missing something that most everyone else's classes have: XP charts. That's because I don't do XP in the normal way in my experimental campaign. Instead, inspired by this document by Beren Kinsman, XP in my campaign works this way:
Characters gain one Experience Point (XP) for every session their player plays them. Characters also gain 1 XP for each 1000 silver pieces of loot they spend. To advance to level L, characters need L^2+(L-2) XP. To advance to Level 2, a character needs 4 XP. To advance to Level 3, a character needs 10 total XP, but already has 4 XP and so only needs 6 more XP. To advance to Level 4, a character needs 18 XP total/8 XP more, etc. Basically, to advance to the next level, double the number of the next level and you've got how many more XP you need.
How does this change my game? Well, I think it makes tracking XP easier, if only because XP is earned in bigger chunks. This particular system also slows down advancement (shh, don't tell my players). For example, according to Beren in his document, it should take about 210 sessions for a character to reach Level 20 using either conventional XP or his system (I'm not sure which edition Beren's using). With my system, though, without spending silver for XP, it will take a character 418 sessions to reach Level 20. I'm still not sure just how much spending silver will accelerate that.
Whatever the case, though, that makes for slower advancement, since I don't think I'm over-generous on treasure when I place it (like when I'm running some One Page Dungeon modules), though I tend to leave treasure as-is in modules I run that already have treasure placed in them. So, if you're interested in a prolonged low-level campaign, you might consider this system.
Another nice effect of this system that isn't exclusive to this system is that players aren't as fixated on killing things as they would be otherwise, but on acquiring loot, since they don't get any XP for killing things. That's still something my players are working on, though. They almost got themselves a TPK last session in a fight with Mad Trees they totally could have outrun.
Finally, not having to worry about XP charts that I don't totally understand frees me up from that worry when I want to make a new class. Someday I'll re-read "Building the Perfect Class" and probably apply it to creating new classes, but for now it's nice not to have to worry about it.
Disclaimer: I downloaded the document "A Level Advancement Alternative to XP" from Berin Kinsman's website a long time ago. When I tried to find the link for it for this article, I couldn't, so I contacted him and asked him if he'd repost it, since I thought it had gone down. He kindly gave me the URL of the page linked above, which apparently had never been down (bad Google-fu skills on my part, apparently). This isn't a big deal, but qualifies as "prior contact" according to my disclaimer down there on the right. It doesn't affect anything, I don't think, but I want to keep myself honest.