As Bucky has said, guilds can span across multiple worlds and even better, you can temporarily Guest visit to a friend's server to play with them or vise versa (party up with them and right click to get the option). Although I have heard that it won't be available right away on the launch date but it will be in soonish (was working in the early stress tests but was kind of broken in the later ones).
Also keep in mind that ArenaNet has said that there will be free server transfers for the first few days on launch so that people can find a world server that suits them.
The only thing that picking a world server is mainly for is World vs World (vs World). If your friends aren't in the same world as you, you can't participate in WvWvW with them.
Since you can join multiple guilds, I'd definitely be interested in joining a Blues one for GW2.