MathCAD 15 has a different idea of "trace" than MathCAD prime, and it is this difference that is preventing me and my group from transitioning to Prime.
The way that trace worked in MathCAD 15, is when you had an x/y plot in front of you, you could right click on it, select trace, and then use your mouse to move the trace around the graph. Being able to use your mouse in this way is very powerful.
This function is very heavily used by MathCAD users (before Prime), and the lack of this function is IMO the main drawback of Prime.
Prime's UI before 4.0 was also too slow to use. It took me longer to complete designs in it relative to 15 because of the response time of the UI. Prime 4.0 feels faster, so hopefully this hurdle is now resolved. Actual computations in Prime were fast, but the UI was very slow.