It's a bit anachronistic to say that WoW changed MMOs to include more soloing content. Soloing in MUDs and MMOs pre-existed WoW.

Wow did prove there's a huge market for different amounts of grouping, and that people who play solo games will play MMOs. And that there is a huge market for tools to bring together people who want to play now, as opposed to wait until their friends can play. And the grouping tools in WoW sucked for the first few years; it was nearly impossible to do dungeons without having your own guild or outside group of friends. It wasn't until later that their match-making tools even could field a group.

Honestly, I'd say guilding corrupts games as it creates a false level of requirement to access content. In an open-world game, people in guilds ignore the remainder of players. They have theirs, and have no need to form ad-hoc groups. And until other players can organize a guild themselves, they're lost and left behind. It's very anti-social.