"And most of all, Haskell makes programming fun!" - If you're doing a startup make sure you choose a language you enjoy. You're going to be spending a lot of time using it.
(Also applies to cofounders and the application itself.)
(Also applies to cofounders and the application itself.)
(and before someone corrects me, yes I know sometimes you can't do that, and yes I know that in that situation you usually have to choose practicality over fun)