I saw Final Fantasy 11 brought up before, but the user didn't go into just how SquareEnix helped solve their leveling quest problems. For starters, FF11 came out before WoW and LOTRO, and is an Eastern game, rather than a US MMO, its key focus was making players work together. You -can- solo in it, but it does not cater to the solo crowd in the least, if you intend to get anywhere in the game, you form a party, join a linkshell, and quest together with people from around the world.
For a while though, there was the issue mentioned in the article, where players would just come across barren wastelands and not see another soul for hours. In a game that keys in on making players work together, that spells doom for anyone who didn't join near the launch of the game. What they introduced last year was a great surprise to everyone involved, the "Level Sync" system. The system works by allowing a fully geared, high level player to join a party with a player of any level range, and activate 'Level Sync', in which their level will be lowered to the highest level of anyone in the group that's in that level range. Their stats and weapon skills, as well as their gear, is all scaled down to that level, and they'll actually receive experience and skill points while partying, since the experience point system scales upward as well.
This way players could play through any dungeon at the level range it was meant for, and get rewarded for it, while helping their friends or even someone they didn't know in the least.
As for WoW, I think it's a shame that so many of the early raids and dungeons aren't even touched anymore. It'd be awesome if a system could be implemented to have scaling dungeons, so you could do a level 80 Strathholme, or a level 70 Black Rock Depths. The loot may not be the top end game heroic stuff, but a token system could still be put in place like there is now, to continue to reward players even if they want to go and enjoy the old instances.