I have been mostly in roleplaying guilds, so my experience is that it has been another layer to enjoy together - besides just enjoying playing the game as in enjoying the game from the game mechanics point of view.
Roleplaying I see as interactive storytelling, enjoying stories together.. the guilds often have background stories, and have plotlines and stories going on that players can join.
So in that sense I do see guilds being sort of "hobby groups", that people with similar interests can gather around, make a social community around that, and then enjoy that particular hobby together with friends. The hobby could be enjoying high level PvE raids, or doing PvP content together .. or roleplaying.. or focus on any other kind of interests that MMORPG's could provide... I'm not sure if there could have been crafting focused guilds in MMORPG's, but I wouldn't be surprised if that would be the case.
But... I think what make a guild a guild .. not just a community, is that people can have a structure as they wish.. some are very relaxed family kind of guilds that are more friendships, some are very structured with lots of rules... And what is most important is that the people joining the particular guild do enjoy just the kind of structure and governance (or lack of it) and enjoy being together with similar minded people.
But also ... I think if we go back to medieval guilds, they were trade guilds, that comprised often of masters, apprentices and novices, that also taught the skills of certain trade like blacksmithing from generation to generation, via this master-apprentice system ... "Always pass on what you have learned", to quote the wrinkly small green creature, Master Yoda.
So it does become down to tradition, teaching the newcomers the ideas of the tradition... so even at times when the old leaders have stepped down or their time has ended, the new people can rise up to be the new masters, and lead the guild on.
My original DAOC guild was still alive .. I think around 2007 or 2008, 7 years or so after the game launch and after most of the original people and original leaders having left the game and moved on already. But the tradition and teaching new leaders the ideology and tradition had kept the guild living .. even though with new people I do assume also the tradition slowly did change to fit the views of the new members.
But still, I do think this is the core idea of the guilds, otherwise they would be just hobbyist communities .. but this transfer of ideas, skills, tradition, and knowledge, is what can make a guild live and prosper for years, and you can trust to leave the guild into new hands, when it's your time to step down.
And this kind of knowledge transfer, well, for us interested in education, it is of course the core of education. :)
Game-based learning enthusiast, virtual learning environments creator, and an avid MMORPG player