My two cents:
If TCP is good enough for World of Warcraft, should be good enough for HTML5 games.
Socket.io is a good solution, but it has its own limitations. Current socket.io plugin for Construct isn't greatest but if you tweak it for your own needs, and respect the limitations of socket.io, it'll be good enough for most projects.
If you really want UDP, currently your only hope is Node-webkit export and UDP / Datagram Sockets module from Node.js. But that's .exe, not HTML5.
Without central server for game logic, there is no talk about some serious multiplayer game. I don't think we'll see some good solution for C2 multiplayer games with the "No programming required" premise any time soon.
Be nice until it's time to not be nice