Allow fractions input and simple operations
It's handy when in Illustrator you have units set to picas and you input pixels just by adding the px, but not so handy when you need to add a fraction and because you don't know you have to calculate the value first, instead of for example typing 1+9/16 and not 1.5625.
Other example is when you're setting guides or dividing grids: you need to break a space in x times or create a pattern.
CorelDraw has it and saves a huge chunk of time.
Egor Chistyakov commented
AI (at least on Windows) allows one operation at a time, and I wish it could allow more. But William says like if it doesn't work for him at all. Is it?