Once the first character reaches a new level, everyone who is two levels below that immediately gets enough experience to advance to the next level. That way, there is never more than a one-level gap between the highest- and lowest-level character. Recetly, I have started giving out “catch-up experience” to other characters whose players participate regularly, but have fallen behind… I’m still working on the formula, but something in the neighborhood of one-quarter to one-third of the experience needed to reach the new “par level” seems about right.

I stat encounters based on the highest-level character. My players are much better at the tactical game than they are at strategizing or roleplaying, so the higher difficulty is not usually a problem.

I went skill based years ago mainly because of this. Your example in ‘test-based’ is similar to mine; “any game where a character gets better at picking locks by killing people is not a game for me.”

And everything is a skill, so everything is a test. Hit points are a skill, and can only gain experience by combat or by literally being hit. All the social skills are, as well. So all the EXP rewards are in the same mechanic, including social.

The reason I went this way, however, hearkens back to the original question…what type of game was I creating (for myself and them)? I play long campaigns, so internal consistency is important. Levelling by Fiat or based on plot also infers that the GM can do it any time, regardless of player action or non-action. The thrill of real Accomplishment is completely watered down due to this. “Drink Roleplay Lite”.
This comes down to how the players perceive their actions affecting their success.
The longevity factor also comes into play when players find their characters getting better based on what they end up doing and based on where the game takes them. I saw a mage becomeing more of a healer-priest, based on the death of the group healer at one point, as well as an assassin pretending to be a priest becoming half-way competent int he priest role. This only happens in a game with organic development and the proper reward structure.

Another issue is the cooperative level of the group. I tend toward an old-school feel, with adventuring groups sometimes not all working together. mmKALLL mentions this, and I have many of the same symptoms often in the game.

I also need to point out that the definition of Metagaming is defined as, ” metagaming is the use of out-of-character knowledge in an in-character situation. A character played by a metagamer does not act in a way that reflects the character’s in-game experiences and back-story.” Metagaming and Roleplaying are mutually exclusive.
Now, it’s a game and has rules, and anything with rules will have some metagaming.

Basically, the more combat centric the game is the more important it is to have equal combat XP (or some equivalent system), the more RP elements and non-combat challenges you have the less you need to do this.