Here are some more:
(1) Retired players import fine.
(2) Make sure you go to game options during league import and click display player development – at least if you want it displayed.
(3) One of the crack beta team members pointed out that you have to manually edit the league names to get the correct abreviations.
(4) You need to check the rules and options for each level in your import universe. So major league rules must be set, in addition to AAA, AA, A, short season, and rookie levels.