For all the attention currently devoted to prospects and with the dozens of cards a player can have before ever actually hitting the majors (and then subsequently getting a dozen more supposed "rookie" cards thanks idiotic rookie card designations), it has always surprised me how relatively little attention is given to minor league cards.
I try (usually unsuccessfully) to pick up as many Pirate minor league team sets each year as possible. Still, there are more holes than full team sets from the Pirate minor league affiliates over the past decade or so.
One of the things I really enjoy is looking back and seeing the players rise (or stop rising) through the minor league system. It's crazy to see how Andrew McCutchen's body type changed and filled out as he advanced through the minors. But that's a post for another day.
When Piratefest rolled around in December, I made a quick run through my excel file to see which cards I had to be autographed, and which ones had already been signed. Much to my surprise, I found two Jared Hughes minor league cards from waaay farther back than I expected that were unsigned. Heck, unsigned is putting it lightly. I didn't even know they existed.