It's pretty obvious that after just a few posts that the term "bust" is probably not a very accurate term to use as a blanket label to describe players. Mainly because it varies and is dependent on quite a few variables. Maybe a number (1-10)/letter (A-F) should be assigned to it to indicate the degree of a bust that said player is. For example, Josh Jackson was a Bust8 or a BustD. With 8 or D being rather high when considering the round and what he gave the Packers in return.
I also think one has to be on the same page as to the 2 ways you are looking at said player to determine bust. Are you looking at it from the Packers perspective and taking into account their investments and returns on player? Or are you strictly looking at it from the side of how successful of a career said player has in the NFL. Because those 2 viewpoints can vary dramatically. Casey Hayward pops into my mind. If Josh Jackson goes on to become a NFL Pro Bowler, I wouldn't change his "bustD" grade for the Packers. Might even bump him up to a bustF, since the Packers not only lost a lot on their investment, but that investment paid off for another team.
I tend to view it from the standpoint of what the player did for the Packers while in Green Bay. Each team is given 7 picks in every draft, wasting any of those on a player that does very little or nothing for your team, is at some level a bust of your investment(s).
So a bad fit with the Packers, multiple injuries or even a career ending injury may not necessitate labeling the player himself a bust, but I would say he was a bust of an investment at some level.
EDIT: I see that Poppa has already started the ball rolling on this while I typed away.