In Guildwars you could join one guild per account.
In Guildwars 2 you can join as many guilds as you like all at the same time pre character.
This is awesome feature, you can join a smaller social guild but still be part of that hardcore pvp guild. Some people think its a insane idea, how can they have popular guild and be the center of attention when there is nothing stopping people joining other guilds. OH NOES
I think its brilliant and another step away from the MMO way of thinking of things.