A lot of these apps have really cool features, but what I miss most from my Ti-89 titanium days back in high school was the readability of the input and output formulas. The pretty print feature becomes increasingly useful as computation gets more complex. Adding these natural language features for things like built in dimensional analysis would really take it to the next level (as things like units were a slow pain on the ti).

Drawing an integral with limits, fractions, exponents, etc on the screen in normal notation provides much easier error checking when your inputs get longer and contain more and more syntax like commas, parentheses, etc.