Consider the following houserule: All characters in the party share an XP pool. Everything happens as before (such as rate of leveling), except whenever an individual character does something to gain or lose XP, it instead affects the pool, i.e. is divided among all characters equally. XP is an OOC abstraction, where OOTS comics even make fun of it, e.g. how you have to kill monsters to learn new spells, or even to become a better baker.
In a party of four, if one character would get 400 roleplay XP, the other characters technically also contributed, by allowing them the spotlight, so the end result is everybody gets 100 XP. Similarly, if one player gets killed, ultimately the whole party is at fault, for not planning, buffing, healing, or whatever in the right manner. If one character spends XP crafting gear for the group, everybody benefits or sacrifices for that situation.
This means players are always at the same level, and always level up (or down) at the same time. If a character dies and is brought back through Raise Dead, the amount of XP they would have lost in standard play is divided among the party members. That means casting Raise Dead may cause no players to lose a level (if they were close to leveling up), or may cause all players to lose a level (if they had just leveled up).
Note linked XP is only recommended with mature players who know each other well. Otherwise players may get annoyed at others who they perceive are “riding their coattails”, or get annoyed at others who make bad decisions, e.g. “stupid, you went and died and caused me to delevel as well!” Of course, that already happens to a certain extent, e.g. “stupid, you went and caused a TPK!”
Many players prefer point buy, and/or average hp on level up, to avoid randomness affecting party balance. Certainly a much greater difference between players is level, where if one player had died a few times and is 2-3 levels behind the party, they may feel behind to such an extent they’ll want to quit the game and enter with another character, or secretly hope or surreptitiously arrange for characters ahead in XP to have an “accident”.
With separate XP, a game is higher stress and people may be (openly or secretly) competitive with other players. With linked XP, the game seems more casual, and more a group endeavor. What other effects can you see in a game with linked XP? Would you play in a such a game?
[Posting in 3.5/PF group since focused on games of those versions, although a similar concept could apply to any roleplaying game with XP.]