I've been creating the same webapp (a simple bug tracking system) over and over again in different frameworks to learn the stuff I care the most:

-Localization (amazing how many web frameworks suck at this, specially, client-side frameworks)

-Learning curve

-Authentication / Authorization

-CRUD, REST, TDD, LoC

-Template language

-Fragments/Tiles/Panels support

-Deployment (i.e. grails WARs are amazingly big)

Over time I've made this webapp in Grails, Play, CakePHP, RoR, Django, Flask, Wicket, SpringMVC, Struts1-2, Hibernate, jOOQ, Angular (ugh!), ExtJS, Express and Meteor.

What I like about this is that I don't have to think about how the product will work, no design, and I can reuse most of the HTML structure (header, footer, menu, etc) and I can compare and reach a conclusion about the framework pretty fast. If I can't make the freaking thing work after two days, the framework sucks or I'm simply too stupid to use it.