After reading a bit of the discussion about the Pirates' drafts, I began to get the feeling that it was a bit short on real data. To address this I took Fangraphs' data for Wins Above Replacement for 2010 and 2011, and asked "How did the most successful players enter professional baseball?" The most successful players ranged from an fWAR of 15.0 (Joey Bats) to -0.4 (Adam Lind). Now, if anybody wants to do this for pitchers, I won't get in the way.
There were 134 hitters listed. Of these 46 (34%) were taken in round 1 of the draft, 12 in round 2 (9%), 6 (5%) in round 3, 5 in round 4, and 28 (21%) were taken after round 4. An additional 37 (28%) were international free agents.
Round 1 is really important. A fail there dramatically hurts the odds of getting a player who will have any impact. You can hope you'll snag a Nyjer Morgan (33), a Garrett Jones (14), an Albert Pujols (13), or a Gaby Sanchez (4), but the odds are not good. Sure, 38% of successful MLB hitters were taken after round 1, but the pool is 40 to 50 times bigger.
International free agents are really important too, and it's here that the Pirates really failed in the before Huntington years. Take a look at the 2008 lineup. Ronnie Paulino was the only international free agent who played a significant role and was signed by the Bucs. For a small-payroll club, that's pretty bad.
In my view, looking at the draft without looking hard at international free agents is really short-sighted. Especially if a big payroll is out of the question.