Wow, I'm surprised how many people aren't interested in clans.
I for one am looking forward to the guild system. I hang around with friends online all the time and being able to form a guild is useful to us for organizing each other (who's in a game/channel, what are they doing, who put this in the shared guild stash, etc.).
I remember back in Diablo 2 I was invited to a guild, and I thought to myself, "Sure! sounds like it might be fun." But when I found out I had to create a new character and put the tag in the name, my thoughts turned to "Seriously Blizzard?". In the end I didn't join because I wasn't willing to create a new character for it.
Besides the whole organization/tags, I think there is a lot that can be built upon with the Diablo 3 guild system. There's the shared stash, which would obviously help store gear your friends might need (it would be cool if you could add notes to each item when you put it in, for example it could say when you mouseover "DO NOT TAKE" or "FOR JOHN" or "ANYBODY CAN HAVE").
I think some sort of Guild Leveling/Perk system could work, but in the state its in WoW, no way. There would be large guilds built just for the perks. So how could that be solved? I don't know. But one thing is for certain: encouraging players to play with their guilds (whether it be arena, or questing/dungeoning) is something that should happen.