Opend20: Character Development
 Character Development
The most functionally superior experience system (from the standpoint of Elegant Simplicity) is something like the one used in the Storyteller system, where 'ranks' in skills gradually become more expensive.
If a 20th level psion decides to pick up a sword and start fighting, why would it take them [the massive timespan it takes to level up to 21] to learn what other, much younger characters can learn by killing a few kobolds? It is true that people get set in their ways, but the standard d20 system is far too oversimplified in this regard.
Of course, that is a matter of opinion, and if the majority of people prefer the current system over a Storyteller-like system, then it stays as is. The current version of OD20 uses a linear (rather than exponential) system for leveling up. In order to maintain a power curve, it is the DC of the skills checks that raises exponentially.
Characters (generally) start out with at level 5 with 50 character points. At each additional level, a character gains 10 character points plus one Action Point. See the Character Creation page for more information on the cost of skills etc.