It's been a long time since I used Mathematica, but I always used it for symbolic computations (e.g. "derivative of x^2 = 2x"), especially when I was learning calculus and differential equations. As far as I'm aware R doesn't even natively have a way of storing symbolic equations, let alone solving them (at the least, it's not its main use-case).
But I could be wrong; I used mathematica and R for very different things so I may just be unaware of their similarities.
In principle R language is equally potent than Mathematica's one (functional+self-modifying), so Mathematica clone can be done in R (and in fact there is a built-in function "D" which calculates derivatives of native expressions symbolically).
Of course this won't ever happen because of the paradigm -- R is for data crunching and does this way better than Mathematica.