"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.)
I was gonna quote exactly the same, to affirm this. Haskell is an absolute joy to program in. Nothing in the code of a Haskell program feels like "extra" or "too much".
It never ceases to amaze me how few people forget to take this into account when choosing technologies and focus strictly on what's practical. The best route is usually to choose something that's fun and practical.
(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)
(Also applies to cofounders and the application itself.)