In 3e and 4e you definitely do not want to have PCs of different levels in the party. That means no level penalties for dying. I'm DMing 3e and I think I'll suggest the idea to the players of losing 1 permanent (non-magically modified) point of Constitution. But that's it really. I've been "de-protagonized" by the 3.5 Druid in my time. It would stop being fun to play my 1/2 elf Ranger if they were 3 levels higher than me too.
In early D&D, the players are playing individually. Every PC begins at level 1, Zero XP. But those early games are (however knowingly) designed for such play. Having this kind of differing of abilities for characters is a diversity not supported anymore. Part of it comes from enabling all classes and class levels to be useful during playing of the game. A high level classed PC player can give a brand new player to gaming, with a brand new PC, a lot of mechanically backed up support. Everything from items, to money, to new allies, information, and even lowering expectations for what the group can handle. Adventuring in a 1st, 1st, 3rd/3rd, 5th, and 9th level group of PCs means averaging your explorations to about 4th and 5th level in the world. Players working together need to choose what they can viably handle as a group and not just individually. Of course, if that 9th level PC buffs his allies with stuff like I mentioned, it really helps him to advance faster too. Also, because of the logarithmic scaling of XP class requirements in earlier editions, it will take every other PC the same or less XP to reach 9th level as it does for that 9th level PC to reach 10th. So the game is designed for balancing classes as well as supporting cooperative strategies.
EDIT:
D&D Next is flattening combat and it will be interesting how they address this.