Tony has the ABILITY to become a great defender, but he makes way too many mental mistakes. For one, his slashing rarely finishes well (how many times does he make the great blow by and gets denied by the rim due to not being able to finish during the chicago series) and his defensive choices are horrendous.
Before calling someone mental, please remember this is the guy the got Ben Gordon on sideline a good 29 feet from the basket, and still tried to contest a desperation 3 resulting in fouling a 89% foul shooter in game 5 of the playoffs. I was there, and sure Paul Pierce had his heroics, but it would not have been needed if Tony Allen had not made boneheaded plays.