Flexbox and Sass-first is a nice surprise, but the switch to rems steals my attention. I know that backwards-compatibility goals kept them on px, but that has long been my one complaint with Bootstrap (as a regular user, not as that guy who "would use it except [minor reason]").
I stayed away from Bootstrap 3 for that very reason in my front-end work.
I would keep my own REM and %age-based "starter grids" (1, 3, 4, 6 and 12 column grids), along with a more simplistic <!--[if IE ]> pixel-based grid (IE = rounding problems), on a USB stick which I would carry around from project to project - allowing me to quickly mobilize from site wireframe to initial mockup with a very short turnaround. For anyone who wants to waste the setup time that I did, I used calculations by-hand along with the "Responsive Grid Calculator"[0]
The other thing selling me over is the _opt-in_ flexbox support as well. If your client has only one big user who is on IE8, and this includes more non-tech companies than I'd like to acknowledge, you will undoubtedly have to support IE as well.
