Every guild tries to make itself unique in one way or another, and yet much of the time it's hard to tell many guilds apart. Most guilds say something like "we are a group of friends" who "focus on casual play" or "on raiding progression" or something which can make clear its priorities in the game, whether in PvP, PvE, or RP.
Roleplaying guilds have a special opportunity to distinguish themselves with all these elements and then some. In addition to raiding schedules, loot distribution rules, and whatnot, they also have a story -- some idea of where the people in this guild come from, and what binds them together. The story theme that binds them may be something as simple as striving to fight against all evil threats to their homeland, or it could be as involved as running a weekly faire, full of trading, performance, and all manner of festivities.
Most roleplayers seem to just drift into an RP guild based on who they happen to meet in the course of their travels and what sorts of friendships they are able to develop. I worked this way for a long time, sometimes successfully, sometimes not, and in the end I gave up, feeling increasingly frustrated that I wasn't drifting into guilds that could really meet my needs. Finally I decided to steer my own ship and I realized that the theme of any particular guild could make a big difference as to whether or not I enjoyed being in it.