[andrewprock]

Feature rich type systems are really great if you have a good understanding of your types. Often, you don't. One of the best parts of software is it's malleability. Pushing business logic up into types makes it less malleable.

[nightski]

Types don't force you to have a concrete understanding of the problem (although they can help think about it). They help you make concrete decisions about your solution. This can save time when changing things often.