I think people need to be really careful about touting the fact that they're casual/social-oriented as if that alone is some kind of magic formula for making a guild last. The problem with that notion is I've seen just as many casual raiding guilds break up/have stupid drama because they *weren't* raiding hardcore enough for a handful of people or because of resentment over mediocre performers being coddled/carried due to being friends with the right people. I've also seen just as many social guilds stagnate and die because they turned into a cliquish, unfriendly atmosphere for new people. Sometimes all it takes is one or two people who are unhappy with the situation in a guild, so I wouldn't rest on your laurels thinking you're safe because you're not the guild screaming 50 DKP minus over vent.

Whether your guild is casual or hardcore in its approach is quite frankly irrelevant to its longevity, imo. There are hardcore raiding guilds who've been around since Vanilla also, and I know it's not popular to say around here but it IS just as valid a playstyle (too often people equate "hardcore" with "complete dicks"; yes, they frequently go hand-in-hand, but not always), and for people who think it's fun, it works just as well as being more laid back.

In my experience, there are a lot more factors that determine whether or not a guild sticks around for the long haul. So I wouldn't necessarily attribute your guild's success to the fact that it's casual. I would attribute it more to the fact that you've got a solid group of people who are generally on the same page about your goals in the game, and that you've got a group of personalities that compliment each other well. Those two things go a long way to making a guild work, regardless of the level of play it aims for.