Disabling WebGL will block a lot of HTML5 games. I think there will be a lot of "WebGL not supported" or "browser out of date" messages that will need updating to include "please turn off lockdown mode"...
<Insert rant about how I miss my Windows 8 phone because it had less crap on it here.>
The only thing I saw in the writeup that I can imagine normal people over 25 missing is web font icons, and maybe emailing PDFs around to sign with iMessage. (Though those come in as jpegs from cameras or PNG screenshots half the time anyway...)
Also, since you can turn it off for specific domains, it's easy enough to re-enable WebGL for some site, while still having Lockdown mode apply to all the random ad serving backends and such you come across. If you're not someone who might be specifically targeted, I think that's entirely reasonable. Secure by default, drop the security level somewhat, by concrete actions I've taken, for some site I want to do something more on.
At some point, I'd assume attackers will try to get people to turn it off so they can attack, but you've made an awful lot more noise by that point.