The things I personally like about Foundation 4 as opposed to Bootstrap (at least in it's current state), are one; it's mobile-first. So it will always be responsive. I've always been interested in supporting as many browsers/screen-sizes as possible, and Bootstrap doesn't push that right now, although in Bootstrap 3 I suppose they will also be mobile-first.

I also really appreciate the mixins, which I feel not only make it possible to create more semantic classes, but also you can easily customize the base Foundation styles with exactly how you want it to look (e.g. Remove a border from a button if you don't want it). As far as I'm aware, it's not possible to do things of that nature with Bootstrap. Bootstrap offers some LESS variables, but the majority of that is for changing colors and small things of that nature.

If you take a look at the Foundation documents, the majority of the pages have a bottom area that describes the use of SCSS mixins. I think they probably do a better job at describing the ways you can use it than I can.

Also, I loved Bootstrap at first. As a developer, it helped introduce me to some design elements like things like the grid and the overall appearance of their elements. I think that's great. But now that I feel more comfortable with design, I think Foundation gets out of your way a lot more when you're looking to do more design work yourself.