First off, I want to start by admitting that I did not read every comment posted in regard to this. By the time I checked in, the forums were way too flooded to read. So excuse any duplicate ideas.
I personally thing the hiring of Juan Castillo is a good move.
1) Most of the other candidates would have had to make a transition to DC just the same as Castillo. I know people say, well a DL, LB, secondary coach should be a good DC. But they have to learn the position as well, teaching technique, and teaching schemes are two different things. Just because you can teach your CB their position doesn't automatically mean you can handle breaking down a teams Offense, and setting up a game plan, and during game making on the fly adjustments. To be honest, hiring one of those other guys would be the same as hiring McDermot again (except you have no idea how these new coaches will work with you).
2) His experience in Offense helps in defense. There is NO QUESTION he has an advantage. He can coach his players to look for weaknesses in O linemen. He knows where they want to go, how to read their adjustments, and they plan accordingly. I guarantee you; they will have more sacks next year. Jan WILL show those guys how to see weakness in a Tackle (maybe he is leaning on one leg or something stupid, and the D linemen can adjust his game to make it more stressful for that o linemen) This means more sacks.
3) He is a known commodity. You know what you have in Juan. He is a well respected, well loved, hard working coach. The dude has a air mattress under his desk. The dude sleeps in the office sometimes breaking down film.
Lets face it guys, every coach has to transition sometime or another. When Andy came to Philly (was he a head coach before)? So he had to learn and adjust. If you hire a LB coach, doesn't he have to learn the coordinator position as well, compound this to the fact that you know little about your new coach. When McDermot became DC, he had to transition as well (even though he did spell Johnson in the past while he was ill). The point is, every coach has a learning curve (EVERY COACH). But we accept it. What makes a DC turned Head Coach capable of calling Offensive plays? But most are very involved (Wasn't Bill Belichick a DC, but isn't he involved on O)? Isn't there a long list of Defensive head coaches who have their hands in on offense in the NFL? But no one complains because they are Head coaches.
Juan is more than capable; he is actually the most capable of all the Joes the interviewed. He was the most qualified, and deserved the opportunity above all candidates.
I have a crazy theory I wanted to put out there. It wouldn't surprise me that Andy hired Juan with other motives. What if Andy is looking at finishing off his contract with us, but after it’s done, he takes a Mike Holmgren like job with us, he would then hire Juan Castillo as his new head coach. Don't forget that Shurmur considered Juan for a Offensive Coordinator job, and now he is a Defensive Coordinator. The dude is obviously well like, respected, and intelligent enough to understand both O and D. He has a strong personality, and will be well liked. I think he will win the fans over as well. My prediction is that in a few years, Juan Castillo will be your new head coach hired by Andy Reid.