While a lot of people who either don't play World of Warcraft, or never group are still wondering what all the fuzz is about, others are beginning to realize the profound changes to the social interactions of WoW that the Dungeon Finder brings. Tipa says: "With patch 3.3, WoW has finally managed to get rid of any need for friends to do dungeons.", and resubscribed to WoW. Spinks comments on that: "I don’t see this as a sign that guilds will die out in the game or that people will stop playing with their friends. ... But I do think that the success of the new dungeon tool will make people ask themselves what they want out of a guild. Guilds are not actually gatekeepers to 5 man instance runs in WoW, although it can seem like that if you run solo.", and concludes: "Unshackling the social side of guilds from the group game may be one of the most long sighted advances any MMO of this generation has accomplished.".
As I mentioned before, the underlying issue here is one of trust. Joining a group requires trust, as either the incompetence or malevolence of the other players in that group could potentially ruin the dungeon run for you. It is easier to trust somebody you know, so a guild group has less trust issues than a group with random strangers. Having guild tag <A> or <B> obviously doesn't make a player smarter or nicer. So why would the player with guild tag <A> be considered a bigger asset by other players with guild tag <A>, but considered a menace in a pickup group by somebody with guild tag <B>? It is just trust that makes us consider our guild mates to be better players than strangers. And as a consequence people preferably group with guild mates, and it is harder for unguilded players to access group content.
As spinks so correctly remarks, that over time weakened the social function of guilds. Especially in World of Warcraft people join guilds for the express purpose of getting access to specific content. Thus a number of guilds evolved which weren't social at all, but only acted as gates to group content. One typical consequence of that is that guilds don't recruit people, they recruit avatars. Your chance of getting accepted into a guild depends on your class and gear, not on whether you are a good or nice player. Another consequence is the death of guild loyalty: While changing guilds was considered a major breach of etiquette in the original Everquest, now people hop from one guild to the next, using the previous guild to gear up to the requirements of the next further advanced guild. If guilds are just the necessary means to access group content, especially raid content, then it becomes easier to see your guild mates as tools towards that purpose, and not as friends.
Now the Dungeon Finder will certainly not eliminate the hardcore raiding guild. But on a lesser level, it is now much easier finding a group for 5-man content, and even raids, outside a guild structure. That eliminates the need to join a guild just for the group content access; and it frees you to join a guild of people you actually like, consider to be friends, and like to hang out with in game. So the Dungeon Finder will in the long run have an effect on guilds, and hopefully make them more social, and less purpose-driven.