I feel it is necessary to balance out my previous post with something more thoughtful. In that line then, I'd like to riff off of it and talk about what makes an MMORPG sustainable, and possible ways to do so in a manner that is fun for the players and allows the developers to concentrate on improving the overall experience, and not just extending it by constantly increasing the level cap.
Now, there is nothing inherently wrong with that model, as WoW has clearly proven it can be an incredibly successful one. I for one am opposed to it as a design model because it it leads to an arms race between devs and players to create and master content, and because it does not allow developers to focus on the quality of a game beyond what happens at the end of leveling.
There is one more reason why I dislike this model, and it is because it follows the vein of normal games that have a definitive beginning and end. MMO's are supposed to be designed to be focused towards player retention and having an end to the content in your game is not consistent with that idea. This leaves developers in the situation of constantly creating new content so their playerbase doesn't get bored.
I think my opinion has been more than made clear already, let's talk alternatives.
Eliminate the Level System - This would be the blunt force trauma way of resolving the issue. Replacing leveling with an entirely skill based system would be one option, but once players reach the maximum amount of skill points available you are left with an end-game in that regard as well. No matter how diverse that skill system would be, you would still need to supply challenges to the player. No levels or skills at all would make player power dependent on the items they acquired. These are viable options but they are more dependent on what the content of the game is rather than using this alone.
Player Created Content - This option could be anywhere from the style of the Mission Architect system, in which PC content is a sideshow, or a sandbox style game like EVE where the other players in the game ARE the content. As I've mentioned before, I'm a huge supporter of sandbox style gameplay and would love to see the style flourish. this style has the advantage of making a tight knit community (in the case of EVE style) and sustaining a game very well due to a dedicated playerbase. However, as we've seen with (what a fellow blogger cleverly named) Monoclegate, this also leads your players to the assumption that the game is theirs, leaving much of the design control out of the developers hands without causing an uproar within the community and destabilizing the playerbase.
Broadening the Game - Instead of adding on top, why not add to the sides? MMORPG's have an incredible tendency to focus only on additions that supplement combat. Except for the First Aid skill in WoW. Honestly, I have no idea why that is still in the game. Tangent aside, instead of limiting a game to PvE and PvP, expand to player housing and allow players to buy and sell property lots. Create meaningful crafting professions that aren't just about making weapons or toys, such as building construction or supply gathering. The point is; you can still have raiding in a game and not force your players to only do that. If that's one of very limited choices for them, they may simply broaden their choices to choose another game.
Gameception - Pun aside, add in some self-contained challenges into the game. The Plants vs Zombies game in WoW comes to mind. Perhaps a Battleground could also take the form of your character as a general in a mini-RTS style, or turn based for that matter. Having one player with a top-down view of the field even in a regular BG would add a whole new level of strategy to the match. You may note that this category falls under broadening the game, but it's distinct enough to warrant its own.
Diversification would be my solution of choice, with a smattering of removing the level system for a skill based one. Player created content, Mission Architect style is not in a place yet to be the center of a game, and I wish Neverwinter the best of luck in pulling it off. EVE Online style sandboxing is also too complicated within the genre to amass any more than a niche following.
Even so, I'd like to hear other opinions on the subject. If you could remove end-game raiding as a main focus of a game and replace it with something else, what would you do? Or would you want to replace it at all?