1) Every class name is prefixed with "uk-": a wise decision to avoid conflicts and make it easy recognise the framework classes.
2) Useful generic classes: UIKit has many helper classes to avoid adding another rule—for removing padding, introducing a small margin, rounding the borders, adding a box shadow.
3) Clean theme: The default UIKit theme looks an order of magnitude better than Bootstrap's.
4) Additional helpful components: Loading spinner, Cards, Notifications, Sortable list. Shipping them by default saves a lot of time in searching and integrating them in your application.
5) Default Icons Support: The icons look clean and beautiful and because they are SVG, they don't require a font file. (Although, requiring Javascript can be turning off for some)
6) Much better components: Try creating an input box with an icon in bootstrap; with UIKit it only takes a little markup.
Whenever I develop with Bootstrap, I have to spend half the time muting the existing styles and introducing new ones. With UIKit, it never felt like a problem.