A few months ago, there were no such things as guild levels, guild perks, or guild reputation (except for on beta, maybe). We had good old-fashioned guilds, without levels or perks, full of players who stuck together purely out of common goals in raiding, PvP, etc., or just to enjoy each other's company. Hopefully, you were in a guild that matched both of those criteria.
Now, we have guilds that can level and gain some pretty impressive perks. Perks include bonuses to experience, honor, justice points gained, increased mount speed and more. In order to gain some of the benefits of guild membership, you have to gain reputation with the guild. This isn't like the old days, when your guild perks were being able to raid or PvP with the guild or when guild "rep" was making sure the other members and officers liked you.
I've been in the same guild around two years now, which is about 80 percent of my WoW history. I tried a couple before my current guild and found some to be too casual (accomplishing nothing) or too pro (full of jerks). My guild now sees all the content when it's relevant, including 9/12 heroic ICC on six hours per week of raiding and with a two- to three-month break from raiding as a guild. I enjoy the mix of the great atmosphere with what we can accomplish.
The guild perks are for me, well, just perks. Even if it took years to hit guild level 25, I'd stick where I am. On the other hand, I know a good raider who guild hops almost bimonthly to ensure he is in the best position for raiding and other achievements. What about you? Are you loyal to your guild for what it is, who the players are, or do you need the perks?