I like the responsive grid. Would like to see some more UI elements but that doesn't really seem like that's it's goal in the first place.

I like Skeleton the best of all the recommendations I've seen thus far, and use it in most of my recent projects.

Skeleton, unlike all the other things I've seen mentioned, has almost no overhead, and pretty much stays out of your way. It's got sensible defaults, isn't as recognizable as bootstrap, and doesn't have any cruft.

The biggest knock on it is that it doesn't have enough widgets, but I for one don't want them. I just want a responsive grid framework that looks decent out of the box. If you're like-minded, Skeleton is a treasure.