Ask HN: How do you decide what to use in building a product?

I got my BS in Computer Science in 2004. I'm now finishing my MBA at Berkeley but I've become very rusty.

In college, I developed some basic web apps on the "latest" technology with a FAMP stack (FreeBSD 4, Apache 2 beta, MySQL 4, PHP 4), some Perl on the backend and light client-side JavaScript. I tinkered some Ajax in 2005 but that's where it ended. RoR, Drupal, CakePHP, Django, Lift or any of the dozens of seemingly awesome frameworks weren't around. MongoDB, Hive (which I'm actually experienced with), nginx, SOAP, REST, JSON, git didn't exist either.

With so many new technologies, how should a developer approach building a new web app (i.e. most YC products)? Which language(s) and/or framework(s) to use?

Maybe this is a whole semester's worth of discussion. But any rules of thumb, rubrics or specific points of comparison would be greatly appreciated!

Thanks, Scott