upside- When drafted vs potential reward. Mccown has great upside because he can be drafted late and COULD put up huge numbers. LT does not have tremedous upside though because he is a top 3 pick, and you will not get better results then that.
I look at upside as where a player can potentially finish the season.
For example Kevan Barlow has tremendous upside simply because he is going to see a lot of touches (350+ IMO). Whereas a Curtis Martin has a much lower upside because he is going to be losing carries to Lamont Jordan in the redzone. Barlow has a chance at being the #1 FF RB this season, whereas Curtis Martin does not.
Curtis Martin could very well outperform Barlow this year. Upside does not necessarily equate to performance, but it is a strong indication of where a player will finish. Unless of course you are talking about later round picks, where upside is the only thing they have going for them.