This has been discussed by a ton of people, and a ton of people that have more experience than me in running a guild or switching guilds.
However, I've been toying around with an idea to form a guild with a non traditional focus.
Historically guilds are formed for a few reasons, but they all have to do with organizing a group. The only real difference is the focus of the group, and the structure of it.
I've spent some time in small, no structure friend guilds, hardcore raiding officer/dictator run guilds, and a long time in a casual PvE guild with officers and friends. Socially, the third was the most rewarding and fun experience.
However, my favorite experience in a guild came from my time playing WAR with Gaiscioch. Gaiscioch has become a large cross game guild with a huge structure. There's a theme, ranks, guild achievements, an in guild trading area, lots of events, all sorts of things. For me, it has grown too large, but Ben has done a fantastic job leading and building this group of people.
Back in its WAR days, Gaiscioch was small, but very well organized. Events were scheduled and kept. Tactics were organized before it began. Players on our server knew when Gaiscioch was out. The best part, is that the time commitment was small and casual. Everything was so well organized that you could join in and immediately be a part of something great happening. We made the greatness happen.
With the Guild Wars 2 multi guild set up, we have even more possibilities. A guild can be built around nothing but its events and tactics. Outside of those events, players go about their favorite path. They can still spend their off time with friends who don't like PvP, or in their Elite World Boss Killers club. But on Wednesday evening from 8pm to 10pm, you are a member of an elite group, practicing sane and deadly tactics in the WvW mists. Or splitting into units and racing through an expert dungeon. Or descending in a wave on a dragon.
Essentially it's a guild based on cross guild events. In the past events have been scheduled on server forums and spread around. But in Guild Wars 2 we will be able to organize them without affecting people's guilds. Admittedly this could be done in the past with Alliances, but honestly I never made a connection with people across alliances. Maybe it was just the way I viewed them.
I'd like to be a part of this Event Guild. And I realize that in order to do that I may have to make it, so I might just do that.