We liked Berlin because it is cheap, has good diversity (I know, Boulder is one of the least diverse places on the planet in contrast), has lots of recent history to explore, public transport is good, a culture that's interesting to us, a decent amount of English spoken, tech scene, startup scene, lots of great places to visit only a short flight away (keep in mind we're from Australia), and most of all it just felt right the two separate times we lived there.

What ended up driving us away was the red tape running a startup in Germany and dealing with taxes, etc. Normally we wouldn't let this deter us, but we had a shortlist of other places we liked just as much.