WoW follow's Everquest's format closely (zones and expansion packs). The only reason WoW did so well was because of timing. Four games were reaching saturation when it was released (Everquest, Asheron's Call, Lineage and Star Wars Galaxies), and a new generation of online players were arriving with cheaper video cards and broadband access. Blizzard also did a great job aiming the release towards the holidays as well and was the first to advertise on television.

I really don't think MMORPGs will be worth a monthly cost until they take on a leveling format like Eve, where you can gain experience while offline, and perhaps allow you to control more than one character at a time inside the same client (sort of like an RTS). Or some sort of other value added feature that hasn't been dreamed up yet.

I think features are the big draw for most online games. They can toss all the fancy cinematics they can render at you, but if it's just tetris with naked women, it's surprisingly not going to sell well.