The big thing I've been headed toward lately is what I'd call "balanced focus." (Or "focused balance.") That is, pick one or two key experiences you want your subscribers to enjoy, and focus your entire design on providing those experiences... but be sure that the actual features that generate those experiences are balanced for breadth and depth.
In other words, first decide how your players should feel when they end a session of your game: tired, exhilarated, pumped, satiated, clever, happy, proud, etc. Then, as you imagine what features can produce the feeling you want, try to balance them within each system and across all systems so that everything adds up to the same effect, and nothing detracts from that experience.
By "balancing within a system" (balancing for depth) I mean trying to insure that both the low levels and high levels of any system are enjoyable, and that they feed back appropriately with each other. This is the kind of thing I'm trying to achieve when I yap about having tactical, operational, and strategic levels of gameplay -- each should be fun in and of itself, but each should also supply something useful to the other levels and depend on resources supplied by the other levels. This internal balancing process makes each system coherent.
By "balancing across systems" (balancing for breadth) I'm trying to describe making the whole supersystem coherent as a persistent environment. Consider the range of systems often developed for a MMOG:
- graphics and sound
- mob AI
- character maintenance
- fear/anger/terror vs. sense of humor
- hardcoded anti-griefing measures vs. player policing
- character advancement vs. complete characters
- classes/levels vs. standalone skills
- rewards for destruction vs. rewards for construction
- XP earned through player action vs. real-time skill improvement
- permadeath vs. cloning/resurrection
Essentially I'm saying I think a masssively multiplayer online game needs to be both a satisfying world and a fun game for as many people as possible. A MMOG isn't just a game, and it isn't just a virtual world -- it's both. It's going to be both lived in (world) and played in (game). So its designers need to try to accomplish both of these goals in one seamless product... and that means having balance, Daniel-san.