Don't design an MMO around the assumption that players are a "type" of gamer. I often hear developers discussing whether an individual is a roleplayer, or a PvP player, or a solo player. Our belief is that while you can certainly find players who exclusively fall into one category, most players dabble in everything. It is tempting to believe that because a player is playing an MMO, and because good MMOs are social games, every player must therefore like to play with other players in a group. Our experience with Guild Wars is that this is an erroneous and dangerous assumption. On any given day, a player may want to play with his guild, or he may want to play with his best friend, or he may want to play alone. The fact that he is playing in a large communal environment is not a predictor of how he wants to play. We should be striving to make games that let you play how you want to play right now, and offer you the flexibility to progress with any combination of players you like.
Don't underestimate the importance of solo play! Sometimes your friends aren't online, sometimes you want to kill 30 minutes while everyone groups together, and sometimes you just don't want to go to committee on every damn decision. The quality of the solo play experience is just as important to the success of an MMO as the quality of the multiplayer experience.