Hey, I love Qalculate!
But I found it a bit strange, that the text input field is just a plain line.
So I thought, why not solve it like this:

Render the formula display below the input on every key press.
But of course do not calculate the result until enter is pressed.
And render that display with an additional vertical bar character at the place where the cursor in the normal input field is.
Now refresh the rendering every second, even if there is no input. (Eg. via a timer. But this feature could even be optional.)
Finally hide the original input field, and make the render area look like an input field, and internally focus the original (invisible) field.

Now you got a *really* cool looking, intuitive input field. ^^
You can make an option to show the original field too, for the cases where one is a bit confused. (But I would leave it off in default.)

How about that? I don’t think there is anything out there, that can beat *that*. :)