1) Standard Award availability - Especially when trying to plan a trip well in advance. (6-10 months out.)
2) Devaluation. (Increasing the number of miles required to redeem an award.) Also, include the status miles being harder to come by with several airlines giving only 50% for the lowest fare classes.
3) Lots of bonus opportunities being thrown at FF members but then not considering that the FF member will one day want to cash them in for a free ticket. (Reverts back to complaint #1.) What a vicious cycle!