Character progression system (=CPS).
I'll try to keep it very abstract, so level-based talent systems (WoW) are covered as well as Ultima Online-like 'skill'-systems. Prejudices have shaped the divide long enough. We should start to focus on what we actually want from a CPS, instead of fighting religous wars.
Too often tools are mistaken for goals. We should first list what we actually want and only then discuss how to realize it.
-> The CPS should give players a sense of character progression.
-> The CPS system should be immersive. This is, it should make sense.
-> The CPS should be intellectually interesting.
-> The CPS should give as little incentive as possible to 'min/maxing the fun out of it'. (e.g. tedious, unimmersive, ridiculous spamming of skills just for the sake of training or advancing).
-> The CPS should not encourage macroing. Macroing should be (almost) impossible.
-> The CPS should prevent a small number of cookie-cutter speccs/skill combinations.
-> Accountability: The CPS should make players treat their characters as an investment. Something valueable.
-> Should the CPS encourage socializing ?
-> Should the CPS have a cap? Should it have diminishing returns and a cap (e.g. 1-1/x). Should it have DR and no cap (e.g. Sqrt(x), Log(x)). Should it have no DR and no cap (e.g. f(x)=x)?
-> Should the CPS also work as a tutorial?
-> What should be the difference between differently progressed characters? (e.g. 10% difference, 100%, 1000% .. ?)
-> How fast should progress be made?
-> Progress decay.
-> Delayed progress accounting (e.g. players become better in intervalls, instead of instantly).
-> Only a limited set of actions grant progression.
-> Only general 'points' are earned that can be invested into 'skills' by the player as he desires.
-> Any form of diminishing returns.
-> The EVE-system: Progress comes automatically with time.
-> Horizontal progression: New 'skills' are added all the time. Thus, there is a theoretical cap that cannot be reached practically.
-> Faction influence on skill gain.
-> Unpredictability: Players only know the rough direction that is required to become more powerful. They don't know specifics.
This is a living document.