jnwi, I agree, keeping a lid on features is the way to go. But surely you can spare a button for the square root /cube root functions which are very commonly used for the sake of convenience? All physical scientific calculators have at least a sqrt key. Anyway, this is just my opinion.
Unfortunately < means "less than" and - means "minus" in maths so using <- as a back symbol can be confusing to a new user. It also looks a little ugly. I would suggest using a proper arrow graphic.