webOS had something called "Synergy" which was essentially a set of databases/schemas/services to provide a unified view on things. I don't fully remember whether this was actually supported, but an app like Facebook could have "its" calendar events (birthdays, events, etc.) stored in one of these and they would seamlessly show up in the main calendar app, without additional setup or sync required. Same held for messaging (SMS, gtalk, Facebook, all in the one app) and probably a few other things.

It wasn't competing with CalDAV, so there is no answer to this question. What it did was present multiple sources, like CalDAV, in a single view.

I never saw webOS's calendar. I'm familiar with CalDAV. Was trying to understand what made it memorably fun to you.