To really make sense of how good the farm system is demands comparison. Baseball America does an annual ranking FWIW. It put the Cardinals 10, Pirates 11, and Nationals 12. Here are BA's top 10 prospects for each team:
1. Shelby Miller, RHP Drafted: Brownwood (Texas) HS, 2009 (1st round).
2. Carlos Martinez, RHP Signed:Dominican Republic '10 Signed by: Juan Mercado.
3. Oscar Taveras, OF Signed: Dominican Republic '08. Signed by: Juan Mercdo.
4. Zack Cox, 3B Drafted: Arkansas, 2010 (1st round).
5. Kolten Wong, 2B Drafted: Hawaii, 2011 (1st round).
6. Tyrell Jenkins, RHP Drafted: Henderson (Texas) HS, 2010 (1rst round supplemental).
7. Lance Lynn, RHP Drafted: Mississippi, 2008 (1st round supplemental).
8. Eduardo Sanchez, RHP Signed: Venezuela '05.
9. Matt Adams, 1B Drafted: Slippery Rock (Pa.), 2009 (23rd round).
10. Jordan Swagerty, RHP Drafted: Arizona State, 2010 (2nd round).
1. Gerrit Cole, RHP Drafted: UCLA, 2011 (1st round).
2. Jameson Taillon, RHP Drafted: The Woodlands (Texas) HS, 2010 (1st round).
3. Josh Bell, OF Drafted: Dallas Jesuit HS, 2011 (2nd round).
4. Starling Marte, OF Signed: Dominican Republic, '07.
5. Luis Heredia, RHP Signed:Mexico, '10.
6. Kyle McPherson, RHP Drafted: Mobile (Ala.), 2007 (14th round).
7. Tony Sanchez, C Drafted: Boston College, 2009 (1st round).
8. Robbie Grossman, OF Drafted: HS, Cypress, Texas, 2008 (6th round).
9. Stetson Allie, RHP Drafted: HS, Lakewood, Ohio, 2010 (2nd round).
10. Jeff Locke, LHP Drafted: HS Conway, N.H., 2006 (2nd round).
1. Bryce Harper, OF Drafted: JC of Southern Nevada, 2010 (1st round).
2. Anthony Rendon, 3B Drafted: Rice, 2011 (1st round).
3. Brad Peacock, RHP Drafted: HS, Wellington, Fla., 2006 (41st round).
4. A.J. Cole, RHP Drafted: HS, 2010 (4th round).
5. Brian Goodwin, OF Drafted: Miami Dade JC, 2011 (1st round supplemental).
6. Alex Meyer, RHP Drafted: Kentucky, 2011 (1st round).
7. Matt Purke, LHP Drafted: Texas Christian, 2011 (3rd round).
8. Sammy Solis, LHP Drafted: San Diego, 2010 (2nd round).
9. Derek Norris, C Drafted: HS, 2007 (4th round).
10. Steve Lombardozzi, 2B/SS Drafted: St. Petersburg JC, 2008 (19th round).
Note a couple of things. Most of these players were drafted quite early. In the case of the Cards, this has been helped a great deal by some excellent Latin American signings. Those typically take a number of years to impact perceived talent levels (or be candidates to play in the majors). Only a few late round draftees (like Mike Adams from that powerhouse, Slippery Rock, or Kyle McPherson, an infielder from an NAIA school) break the top 10. In addition to some very nice international signings, the Cards have not promoted players who could have played in the majors.
Washington gets a high ranking largely because of Bryce Harper. Almost by definition a system with Harper in it is an excellent minor league system, because he projects to be a star. Most of the rest of their players are unproven, or less than major talents. BTW, Rendon is hurt again. The Nats think he'll play this season.
The recipe for a good minor league system is to draft early and often, work the international scene hard, and don't promote players until they force your hand. The Bucs have drafted early (but not typically multiple times in the high rounds), stopped looking internationally for the most part, and promoted out of desperation. Were it not for the Moskos draft, one might almost argue that DL was starting to get it in 2007. Ah well.