As far as the type of product and framework usage, think something like Facebook (it's not exactly a social network, but close enough for evaluation in this context).
Basically, I'm just looking for something robust, scalable, easy to work with (small learning curve is a plus), compatible with older browsers, and well integrated with other technologies (e.g. Postgres, unless there's a compelling case to be made for Cassandra?).
Other frameworks/tools I've looked a bit at or been recommended:
Google Web Toolkit + Server-side Java
Django
Ruby on Rails
ASP.NET + Mono? (I know...)
PHP/Perl/BBQ
I don't have a whole lot of experience with Web frameworks, so no matter what we end up choosing (whether I've mentioned it or not) I'll be learning something new. Any thoughts or recommendations? Anyone have any experience with Nagare (or Pyjamas)?