I am not sure what "mobile first" means, could you please explain what that means for the end-developers?
The only thing to that effect I see right now is that responsive less files will no longer be separate from the core. That will probably mean that I will be forced to support mobile platforms.
All mobile first means is that instead of scaling CSS down for smaller devices, we start from there and work our way up instead. So far it's smarter defaults, little bit less code, etc.
That should have read "forced to deal with mobile platforms".
I currently have scenarios where I want the same version on the desktop and mobile. That's trivial because I simply don't include the responsive CSS file.
In 3.0 the media queries are part of the core. So I can't just leave the responsive CSS out. Further, it's mobile-first so things like the navbar would be collapsed by default on a mobile device. I would be forced to negate that to get an uncollapsed navbar on a mobile device (as desktop version).
But the point of mobile-collapsing navigation on mobile devices is for a better user experience. Why would you go out of your way to ignore that right away? And if you really didn't want it, you could always remove those lines of code from your file. Or comment them out so it's there when you revert.
The only thing to that effect I see right now is that responsive less files will no longer be separate from the core. That will probably mean that I will be forced to support mobile platforms.
Is there more to it?
(Aside, wishing someone would take a deeper look at https://github.com/twitter/bootstrap/issues/4935)