The real amazing part of Seaside is how is uses continuations to turn the http request/response cycle inside-out. HAML doesn't appear to do anything like that, so comparing it to Seaside seems misguided.
But apart from continuations seaside also provides system to abstract out HTML generation, that not only changes syntax of HTML but also has support for building page from tree of prepared components (like you would construct desktop application). And this HAML thing only seems to change syntax of HTML and not add any similar capability, so comparing it to Seaside probably is misguided.
I think the real point wasn't even that Seaside's HTML generation is smart/powerful/fun. It's this:
I enjoy programming in Seaside because when I'm programming in Seaside, I'm programming. I'm not wrestling with markup; that's for computers to do. I get the same pleasure out of using HAML.
Anecdotal counterpoint:
I'm one of those who has the opposite experience with Haml. It's a constant fight. OTOH, XML/HTML is stupid easy; my editor takes care of the annoying parts, and I never fight with cryptic errors. It's easy to cut-n-paste for assorted sources, and use WYSIWYG editors if and when I want to.
I think I could live with Haml (I'm sort of compelled to deal with it since I took over an abandoned Rails project) if it did not use significant indentation. It works ok for small chunks of text, utter pain when there's much nesting going on.
The application of the description "terse" is quite on the money. It's not a feature.
xml.form {
xml.label "zipcode"
xml.input :type => 'text', :name => "zipcode'
xml.input :type => 'submit'
}