As gamers, we generally want the games we play to do it all. If a new MMO is slated to come out, it should have awesome PvE, and PvP, and RvR, and a moddable UI, and instances, and open dungeons, and huge zones, and strong story, an interesting endgame, no grind but not too fast, lots of mounts, crafting, so on and so forth. While a game that does everything extremely well is certainly possible, it's just not generally feasible. Game designers are in the unenviable position of having to create as many of the systems that players as want as they can, with as much quality as they can manage, without taking too long or going over budget. Since time and money are limited, the chances of crafting a perfect game are almost nil, and the designers have to pick the systems they believe are the most important for their game. Sometimes they get it right, sometimes they don't.
I'm curious though, as to what the core "must-have" features of an MMO really are. What features, if omitted, would result in gamers avoiding the game entirely. PvP has become one of those "go to" features that developers (or maybe publishers) seem to insist on including in every new release, but it seems like more of a buzzword than a required core feature. If, for example, Rift had no PvP whatsoever, it might have fewer subscribers than it does, it could possibly even have more, but the lack of PvP would hardly have prevented more than a minority of players from buying the game at all.
Of course, the type and style of MMO also has a major impact on this. An easy answer to the question would seem to be "PvE content", but even that isn't true. Many people consider World of Tanks an MMO (I'm not really one of them, but humor me here), it has no PvE content at all, nor even a persistent world, yet it's certainly a successful game that lots of people are playing.
The more I think about it, the less sure I become that it's even a question with an answer (and the more I pity the poor game designers trying to figure out the secret sauce of MMORPG success).
It seems like the strongest games are the ones that take a concept and build the game out from there. World of Tanks is such a game and has a pretty simple concept - online tank battles with WWII era tanks. Features get built out from that concept, rather than the other way around. Games that build from a feature list seem to end up weaker. I suspect Warhammer Online is an example, as all through development they talked about features. "We'll have this, we'll have that, we're adding new features left and right!" Rather than a single defining concept to build the game around, Warhammer Online was a collection of features/systems shoved together into a single bundle. Strangely enough, I suspect Dark Age of Camelot was the other way around, with systems and features created to support the core concept of Realm vs. Realm combat. Warhammer Online didn't know what it wanted to be, other than "like WoW, but not WoW, with RvR too!"
Yes, I realize I'm just rambling now. I did give this blog its title for a reason. I'd love to hear people's thoughts on this -- what do you think the truly "must have" features of an MMO are (if any), and would you rather play a game with a large number of loosely interconnected features that offers a wide variety of gameplay options, or a highly focused game where any features take a back seat to the concept behind the game?