I will be interested in the 'feel' of Donkey Kong Jr, I've been playing mine since at least 1986 (I found when I took it apart some years back, that I wrote my name and year in pencil on the PCB!).
But just because I've played it a lot does not mean I am great at it though
I don't know if it the logic or 'muscle memory' but on Donkey Kong JR I can move the character along fast. On Donkey Kong (which I only recently got) it seems the character is locked to the 'beat/clock' of the system. On DK Jr it seems you can move in-between the 'normal beat' and move fast over say 3 character spacings.
Interesting to hear from people who know Donkey Kong well, if this is also the case, and it is about timing of button presses. The difference in control pads between the 2 games probably makes a bit of difference.