Fundamentals such as tackling, have been extremely poor. Don't know why, maybe it was just to try and limit injuries (and to be fair, they have been hit with some major ones, especially on D), or I don't know what.
OF course, maybe it was all a Strategeric Dungy rope-a-dope to lull the other teams into a false sense of security regarding our D. :-))
I count lack of execution under lack of ability. Maybe on paper they had the ability to execute the gameplan but on the field this season they didn't show much ability to get the job done.
I think the biggest change for the Colts defense was in Freeney. Historically (taht being his entire career up until the Wild Card game this year) Freeney over commits to rush the passer and allows plays to pass him, he frequently has his back turned doing his spin move while RB run past him or QBs scramble in a direction that were he facing them he could stop them. The last two games Freeney has finally found some discipline as a pass rusher, he now actually looks to see if it's a run play and does stuff other than the spin move to try to get to the QB. Obviously it's wasn't all on Freeney during the regular season and the improvement is all because of Freeney now, but when looking for the single most obvious change it would be him and that could be symptomatic of an overall change in the Colts defense: more discipline, less over committing, more figuring out what the play actually is before trying to stop it. And now the question is will the keep it up.