I'd add one thing, though about an optimized stack for a startup with a lot of unknowns. I've attended a couple hackathons and such and seeing how much traction and how quick small teams are able to piece stuff together with a JavaScript/Node.JS/MongoDB stack is pretty staggering. Not sure if it scales far down the road or what issues people will run into, but having a single language across the entire stack seems to have some productivity benefits. Especially when you have a developer that has to work in two or three of the layers (UI/Server/DB) all at once...
There was some VC around here who talked about the advantage that startups have because they can choose some of these tools to quickly develop in and such...
Right now I skip from writing heavy JavaScript in the UI (AngularJS) to writing Python on the server. It's a context switch, how I iterate over collections, parse strings, etc. is totally different. I'm not in love with Node.JS, but love the idea of a single language/code constructs throughout the stack. Especially in a small-team startup environment. Maybe someone can come out with a synchronous server-side JS engine? :)
Just realized that it's also a context switch when I go from Python to Mongo, especially when I'm working in the Mongo shell.
There was some VC around here who talked about the advantage that startups have because they can choose some of these tools to quickly develop in and such...