The Pirates have seen a slow and gradual build to their farm system over the last 7-8 years. For some, this build has been too slow, although that kind of pace is expected when you spend so many resources on very raw prep players. They've seen top guys get promoted to the majors the last few years, such as Gerrit Cole, Starling Marte, and Gregory Polanco. Those guys have been joined by players with smaller roles, like Jordy Mercer, Josh Harrison, and Jeff Locke.
You may have noticed a trend in my early previews of the 2016 minor league rosters (if you missed them, you can read the pitching, outfield, and infield articles at those links).