I am still stinging after landing Francisco Liriano in my AL-only Yahoo! autopick draft. As a result I have been examining the Yahoo! rankings more closely. For mixed leagues, here is a list of players you should exclude in all standard Yahoo! autodrafts (21 rounds - 9 hitters, 9 pitchers and 3 bench players.) In this set-up you will need to pay extra attention to any players ranked higher than 253 (21 rounds * 12 teams) and 210 for 10-team leagues. I have a 16-team league, so I went as high as 336.
None of these players are out for the entire season, but all of them do not appear to be available to help fantasy teams before June at the earliest. That is too long to merit a high pick on them. However, you could pre-rank you players and tag them on to the end of the list.