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)?
As the lead developer of Nagare, I really encourage you to try it in real on your product, which is the best way to see how Nagare is truely different than the others frameworks like Django, Pylons or Flask. Nagare is components oriented (it shares the same components model than Seaside) and its set of advance features like direct callbacks registration, stateful components, Ajax without to write any Javascript code or the use of continuations makes a Web application looks like a desktop one. In fact we have often found that developers like you, without prior Web experiences, can be quicker to get Nagare because they have nothing to "unlearn".
Talking about reliability, scabability and compatibility, you can check some of our important projects in production today.
For more info, don't hesitate to ask and share your experiences with us.
I would suggest Django + Pinax. Both are robust and have less learning curve (if you have familiarity with Python).
This should have you a social network up & running within a day or two.
For the front-end use the usual suspects. javascript, css, html. I believe there are some terrific libraries for javascript.