Ruby is really good at writing DSLs, Javascript sucks at it.

I'll happily admin that this is user error, but I always have trouble editing Gruntfiles when I start to bring in other packages. Adding an additional task to a Gruntfile is way harder in than Rake, or make, or even Ant for that matter, mainly because you are dealing with hand editing a huge JSON blob.

Tweaking the file to add a different path in or whatever is fine, but there are many times where I wanted to to copy and paste something in to make it work, and I spent as much time fiddling with the format of the js file as I did solving my problem.

A lot of people love grunt, but I'm not one of them.