Cataclysm is focusing on many new things and changes to the old ways of doing things. With guild experience being on the forefront of these changes, it is no surprise that suddenly people are taking more notice of their social environment, as it may have an impact on the game once the grind for your own guild goes live. Having a group whose personnel and personality are on the same page as you might be nice if you're planning on receiving some of the benefits (like mounts, special patterns and bonuses to rezzing) immediately.
Some people are very focused on guild perks and have assembled elite crafting and leveling forces or are maintaining the same strict raid teams they've had for a while. Others don't care as much. Blizzard has made it clear that all sorts of guilds can effectively benefit over time from the guild leveling system.
Our guild is especially looking toward having enough people to be able to level up quickly, even if we won't have a bulk of our guild XP coming in from raiding achievements. This bodes well, as we've very quickly started taking on orphans from other servers, alts of people in other guilds and raid teams, and people rolling up new characters.
Does your guild care about the XP system? Are you a close-knit, small guild that will be leveling up slowly, or are you a large raiding guild that things like mass-rezzing appeal to? Or is your guild looking forward to something else entirely?