Let's start with the work done by John Sickels.
Sickels continually talks about percent over league average as a barometer for a successful or at least interesting prospect. Well, Selsky was more than 25% better than league average in OPS, BA, OBP, and SLG. His HR rate and SB rate are also well above league average.
He did the same thing in the Arizona League last season (though, admittedly, in only 20 games).
In other words, he has shown many of the earmarks of success we should be looking for in two of the three leagues he's played in since becoming a professional. (SSS, of course, applies in any of these discussions.)
Very few Red prospects have done that-- and all of those that have done it are 20 spots or more ahead of Selsky in this particular prospect list.