Cameron called attention to how little incentive MMO players typically have to return to earlier adventuring areas, as well as the importance of "coming home" in hero stories.
This article was getting all jumbled, so I'm going to save the "coming home" bit for another day. Instead, I'm just going to tackle the problem of beginner areas becoming obsolete to players as they level up. A feeling of "home" can really help, but let's talk about that later.
Past games have included or proposed methods of what I call backsliding. Backsliding is different from deliberately returning to past areas because the player is forced to give up something and may not even want to go back. Some examples:
A character wipe waits until the player has progressed far before sending him back to the beginning without a character.
The cancelled Trials of Ascension proposed a system of limited lives (delayed permadeath, you might call it) and enough dynamics to keep replay fun. The player generally would not progress as far before being returned to the beginning, and individual player-characters would be wiped at different times due to the circumstances of their individual adventures. The world was stuffed with enough dynamics (including the results of player actions) that areas might feel different a second or third time through. They really had a lot of interesting ideas. I recommend browsing through this fansite's quote database in addition to the official site.
EQ-style backsliding allows players to lose levels, falling back just a step at a time with only the same familiar ground to be regained.
Richard Garriott's team has come up with an interesting model of character saves for Tabula Rasa, allowing the player to save a particular build of his or her character and return to it later (ex: go back and play your character before you chose one sub-class over another, thereby allowing you to explore alternate gameplay avenues without returning to the very beginning).
Another failed gamed -- I think it was Realms of Torment (later known as Mourning) -- proposed a bloodline system. Your character would have a limited lifespan, but certain traits and heirlooms would be passed down to your character's offspring. Trials of Ascension similarly allowed players to sacrifice their last remaining lives to create an artifact, which would appear randomly in the gameworld to be acquired by some other player.
In previous games, it seems that players seldom return to earlier areas without penalty for reasons other than social reasons having little to do with the game's design, such as helping a friend level up.
So what can entice the player to return to areas and really enjoy it?
In the past, progressing through a faction's available quests and other content occurred in a single area. You work your way up the faction ladder in a particular town by finishing quests focused on the surrounding area, and then you move on to the next (higher-level) town to work your way through its questline.
But what if there was no immediate jump between the guard captain's quests and the king's quests? The game could encourage you to move on to other areas, but later encourage you to return for different questlines/causes. If it's a level-based game, then the town might have quests for levels 1-12, 25-34, 56-70, but none for the levels in between. Of course, this requires an EQ-style cohabitance of low-level and high-level content. EQ's Oasis had both lower-level ghouls and higher-level sand giants, and players loved it...even as (because) they had to watch their backs and occasionally run for their lives. Breaking up faction progressions like this also helps to create an impression that total progression isn't easy. It's an adventure, a journey, that was months in the making.
There's also the Oblivion model. In that game, some factions exist in every city (Mages Guild, Fighters Guild, Thieves Guild) and progressing through those faction involves moving between cities frequently. You might complete only one quest (in an MMO, I would expect it to be something significant if it's the only one) before being directed to a faction official in another town. Encouraging travel this frequently has the benefit helping the player see the big picture (the world's lore on a macro scale) and can help keep gameplay fresh.
New visions and opportunities
In real life, returning to a place we haven't seen in many years can feel nostalgic, but we can also be surprised by all of the things that have changed. Or we might be surprised by all the things we didn't notice before or don't see in the same way, because we too have changed and perceive the world differently than when we were younger.
Obviously, the sort of dynamic worlds that MMO gamers keep hoping for might be capable of offering the player such surprises on returning to old territories. Not so obviously, some of those changes could call for input from advanced players. Consider the classic tale of Robin Hood, in which King Richard is off in the Crusades and the wicked Sheriff of Nottingham oppressing the people and trying to usurp the throne. In an MMO, this would translate into a situation of a once-content but now-oppressed town that needs the help of advanced players working cooperatively to free the people. Such situations don't have to be so dramatic and expensive for the developer, of course.
As for seeing the same place differently, that can be accomplished largely through dividing high-level and low-level content more sharply in towns. In real life, odds are that you noticed stores and venues at age 25 that you didn't notice at age 10... because you didn't care about the same things at age 10, and didn't have access to the same things. You might be more aware of the names and locations of pubs, for example. Similarly, if particular places and NPCs have little effect on or potential gameplay opportunities for players at early levels, then that area will feel different when they return to it at higher levels. Perhaps the NPC nobles or other leaders are only interested in higher-level players, for example.
NPCs can be revealed for very different characters than you originally knew, through knowledge the player picks up in other areas. Perhaps the village drunk seems harmless and really a good guy when you first meet him, but you learn later and elsewhere that the drinking is just a cover for the espionage he's performing for the hated raiders who sack the town every so often. By acquiring that information, it might open up new dialogue options and quest options with him. After you've confronted him, he might look at you differently as you pass by, scowling whereas he used to smile and cheer you with his beer held high.
What else might make earlier areas worth coming back to?