The Verge has posted an in depth overview on what WebOS could have been on their website.

There were some really nifty UI ideas in the unreleased WebOS updates, like the new Panels feature, an expansion on the original cards idea. It's a nice way to open a link in a mail without leaving the mail application by sliding a webview halfway on the screen, and leaving the original message, and thus the context wherein the link lives, visible.

I never had the chance to use WebOS, but it seems to have been an awesome platform that never really left the conceptual phase.