In most MMORPGs, older / lower level areas get abandoned, but why should they be?

Landscapes that were once populated and rich with activity become quiet suburbs where the NPCs are left to themselves. As episodic content is added (especially full expansions), players are bound to migrate to the new areas. But it's more than that.

I suspect that game developers tire of content even quicker than players.

Let's face it, most modern MMORPGs are fueled by Achievements on the old Explore | Achieve | Kill | Socialize paradigm (The Bartle Test). It's difficult for developers to keep up with new content demands for Explorers anyway, so even though that's the largest segment, it's not who MMORPGs like World of Warcraft cater to. There is also palatable affection among players for older areas that's left unexploited by game developers.

Player control of leveling rate

I can think of a few mechanics that MMORPG developers could apply to revitalize older areas and content, but this one comes out on top for me: Allow players to pause their level rate by sitting at any specific level and banking their exp until they want to move on.

Imagine you and your friends being able to keep your characters at level 18 as long as you'd like. You could explore Deadmines or Blackfathom Deeps more completely, with everyone in your party being the exact appropriate level, something that happens so rarely since leveling at that stage goes by so quickly.

Effectively, any given level could be treated like "Endgame". Gear could be more varied, allowing players to completely "conquer" a level by collecting superior or epic gear.

The situation now

Right now as it stands, after an MMORPG has been established awhile, the pressure for Endgame is so strong that players begin to demand skipping the entire leveling process. I would not be surprised if Blizzard announces all classes can skip to higher levels for their next expansion, not just the planned Deathknight class. WoW in particular is a competitive game and that's much of what current Endgame is about: lower levels don't really participate in the competitions that "matter".

What a shame to abandon places like Elywnn, Ashenvale, Redridge, Duskwood, Desolace, etc. Some of the early maps exhibit the original tender loving care by the developers that hasn't been matched since, the content in some of these places is clearly superior to what's come down the pipe lately. The developers may be too bored to revisit these places, but I bet a lot of players would love to camp out in Duskwood for awhile, especially the Roleplayers.

Players could be savoring and deeply exploring the entire world of Azeroth (and other games, this hardly applies to just WoW). The world could be more evenly populated, feeling much more alive and real. Sure, most players would still prefer to be up there at level 70 or whatever the most powerful level is, but I think if they enabled the ability to explore all of the content at the player's own pace, they may be surprised about how many people want to live in their world a bit longer.

I can't take credit for this idea, it was used in numerous MUD designs, which is why I'm surprised we haven't seen it applied to MMOs much.