I run the Eagles blog here on SBNation, I've seen every game Sean McDermott has ever coached... here are my impressions.
The last two years have seen a serious decline in the Eagles defense under McDermott. Now, how much of that is fault and how much is on the personnel is certainly up for debate. He has dealt with some injuries, not as many as people seem to suggest, and the fact is that a lot of the defensive personnel just isn't very good. Part of that could be on his inability to develop them, but then again you can't expect the guy to spin yarn into gold. Some players are what they are.
So this makes it hard to evaluate McDermott. There's really very little evidence to suggest that he can do a good job, but that's not necessarily proof that he's bad. I sure would have liked to see what he would have done with a better secondary...
I think what the Eagles didn't like was the way he handled the adversity on the defensive side of the ball. He would get overly cautious at times, his blitz timing was questionable, and he was constantly shuffling the lineup and trying different combinations. The shuffling never bothered me because I was ok with him trying to find something that worked, but I can imagine how players would be frustrated when there's a new starting linebacking corps every week(that was literally the case in 2009).
My biggest problem this past season was the inconsistency in his play calling. We all expected him to be an aggressive, blitz heavy kind of guy and he wasn't always that. There were times when he seemed to go into a shell. Maybe he was afraid of exposing his sub-par secondary, which is a fair concern, but it's not as if the passive approach was working either.
His defenses did get turnovers(like they always did under Jim Johnson), but having Asante Samuel leading the NFL in picks year after year certainly helps that. Also, I hated his stubborn philosophy of not matching up your best corner on the other teams' best WR. McDermott has his corners play a side of the field and let's the offense dictate the matchups. That's ok when you've got a few good corners, but when you let the offense pick on a guy... that makes no sense to me.
The Eagles allowed a franchise record 31 TDs passes this season and had the worst red zone defense of any team in 20 years. I think that in the end, those were the two stats that killed McDermott. Like I said, personnel was a part of that... but you don't get to be historically as bad as they were this year without ineptitude at all levels.
The book on him is not closed by any means. This is still a guy without much experience as a coordinator. There are notable personnel issues with the Philly defense. Basically, he hasn't proved that he can be a good defensive coordinator, but I can't say as though he's proven he can't yet. The reports are that Reid actually fired McDermott back on Wednesday, but has been trying to help get him a job this week before letting everyone know he's been released. He still likes McDermott and thinks he can be a good coach, but seems to feel the Eagles are in win now mode and can't wait for a young DC to learn on the job.