Also of note is that Seaside uses the concept of continuations to manage multi-page flow which is a very fitting abstraction for this problem, and that most languages can't afford to do. That means they can code complex behavior as if the line between client and server did not exist.

Kind of, but in the 3.x series, please note that continuations are now completely optional.

As continuations are quite memory heavy, the new way allows for lighter sessions.