Hibforlife, contributes WoW's slower growth rate to the fact that the developers decided to build the game around raiding and guilds. I would admit that prior to TBC, WoW was much more of an open ended game. Players did not need to raid to be able to compete in the world, in fact dungeon gear from Scholomance, Upper Blackrock Spire, and etc were good in the open world, even for pvp. But what made Vanilla WoW so great was that the gear gap wasn't so large. Well it was, but since very few people raided successfully, you were less likely to encounter a player who would vastly out geared you.
The interesting thing about Vanilla WoW was that many players explored the world and world pvp was a common occurrence. Though, the reason for this was probably due to the fact that at the time there were no flying mounts, everyone had to run on foot or on their ground mount. But nonetheless, many people didn't focus on the raiding part of progress, but rather the exploration and social part. And no you weren't always forced to group.
Unlike in TBC and other subsequent WoW expansions, you were forced to participate in one or two aspect of the game, pve or pvp. If you did not keep up with your guild you were left behind, meaning that when it came time to raid, chances are you won't be invited because you did not spend as much time as your guildmates accumulating your level, skills, gear, items, or even gold (for repairs). Soon enough most of the playerbase was forced fed into keeping up in pvp or pve. As a matter of fact even the pvp changed. After TBC with the introduction of pvp gear and arenas, the progress for gear became ever more reliant on a competitive group. Needless to say many were not able to aquire the best pvp gear.
So in the end, I agree with Hibforlife. I think when developers focus their game design on guild participation, it forces people to play in a way they may not want to. Most players like to play solo, which sounds ironic giving the nature of an MMO. But because everyone have different goals and interest, most just want to sign on and do whatever they wish that will lead to some form of progression without all the interruptions of game design, meaning not be forced to drop everything and help the guild defend a base or raid. I do think a lot of players leave these type of MMOs because they often get left behind and find that there are no other ways to enjoy the game, at least for them.