First: Clans are what you make of them. They can be cliques, they can be snobby, they can be collections of annoying fanboys.
Or they can just be that group of people with similar playing interests whom you know you can depend on to host and play games you like.
Second: The WC3 online scene is dead. You're 7 years too late for the good part. It's dominated by mass-spamming asian DotA bots, and it's just crappy. From about 2003 to the release of WoW, it was the best place on the internet. Imagine a gigantic game engine with thousands of features and options available to hundreds of thousands of active players in a completely customizable format. THAT was what made WC3: the custom games. Not that the normal multiplayer wasn't highly entertaining on its own, but the custom games were like crack. There were large numbers of people who literally had a life-harming addiction to that stuff.
Hundreds of user-created games, often complex and in great variety, 24/7. It birthed entire genres (like tower defense). I'm very interested to see if and how SC2 surpasses it. They already have a plan to sort the custom games into different lobbies, completely fixing the dota/spambot problem.
EDIT: Reading that, you'd think I liked WoW or was suggesting it is the new best place in the internet. Quite the opposite. It killed the WC3 online community overnight, ending the gamers' utopia. The spambots killed it a second time after the WoW craze died off and players were coming back.