I wish more emphasis was placed on mental math. I've tried to encourage my daughter to do more of it, but without much success. Next year she'll be in algebra, and I think it'll be tough for her.
I can't begin to tell you how many student workers I've had who needed to write out things like 28-8=20 in order to write a score on a paper they were grading.
Eons ago when the sales tax was only 5% my dad could usually figure out the total for the bill in his head before the chick at the register could do it with her chart and cash register. It was pretty simple, really, yet they were always amazed.