Being in a guild brings such an elevated purpose to playing WoW. If I wasn't in a guild, I honestly don't know what I would do. Not only as level 80 (wtf how would I raid?) but even my lowbies. The alts I play most are the ones in a guild. I play an alt on an entirely different server where I am the only person I know that actually plays regularily, and I have so much fun because I'm guilded on that character.
Imagine, for a moment, that guilds didn't exist (and PuGs didn't suck, so you could still raid). What would the game be like? Would trade chat still be the seething cesspool it is now? Perhaps. Would more people use their friends list (and for that matter max it out)? Certainly it would be harder to get groups, etc., but at the same time would it still be fun?
In real life (yes, it exists) I am very socially inept. I'm not comfortable making friends, I'm very shy, and I won't talk to someone unless they talk first. I never initiate conversations, not even on MSN. In WoW, I am, quote Pallymar, "the most outgoing person on vent". I can have active, productive conversations, I can enjoy being with other people, even in a virtual setting. I make silly jokes, contribute to off-colour discussions, and generally make an ass of myself and have fun doing it.
This in itself is a revelation for me. And, to return to the point, none of this would be possible without a guild.
My first guild was None Shall Pass. NSP was (and still is, though membership has unfortunately much diminished) a guild composed mostly of RL friends and a few in-game friends. It was all levelling, PvP and social. No raiding whatsoever. I actually joined NSP by accident ^.^
One day, in my low 20's, I was running around through Ashenvale. Suddenly, I find myself dead, ganked by a horde. And as I'm about to release and run back to my corpse, I notice that he's camping me. Well, crap. So I sit at my corpse, planning to either wait it out or just log off when a group of Alliance come up the hill. Our guild leader Blackdog (70 at the time) was running a few of the lower level characters through some quests and happened to be in the area. He quickly dispatched the horde and the priest rezzed me. I waved, thanked them for their help, and was about to continue on my way when BD sent me a ginvite.
Now, I was honestly hesitant to accept it, since the last one I got was a "OMG I GOT THIS MANY RECRUITS PROMOTE ME NOW!" guild. But I figured, eh, they seem helpful, and joined up. And stayed with NSP for almost a year.
I loved NSP. It was a great environment, everyone was nice and talkative, and they became my little circle of friends on Stormreaver (especially after some drama with my old roommates). I levelled straight through to 70 with them, and was loathe to gquit, even for a raiding guild. I passed up on tons of endgame in TBC because I couldn't bear to leave NSP.
Eventually, I left for a raiding guild on Stormreaver that was pretty much a grown-up, raiding version of NSP. I still kept in touch with all of the NSPers and ran instances with them all the time. In fact, just before I transfered to Bonechewer to join Ascended, my friend Xyliaj and I ran one last heroic together, and I actually delayed my transfer by a few days to wait for her.
Now, in Ascended, it's like a family. A crazy family that you wouldn't necessarily want to introduce to your boyfriend, but a family nonetheless. And it's awesome. There is a coherency to the group that makes us work real well together. The guild has its jokes and its serious moments. We talk about boobies in mage chat one minute, then the next we strategize. Hunters talk about MDing Vic, but hunters are... weird. To summarize: Ascended makes playing fun. And unpredictable.
If I hadn't had such an awesome group of friends on WoW, I probably would have given up ages ago. In my opinion, you miss out on so much if you're not guilded. It is an absolutely necessary part of the game, and one which makes it all the more enjoyable.