I thought it was pretty messed up when they canned Darabont. That said, the pilot was amazing and everything after was meh. Then AMC cut the budget despite it being a hit for them.
Mazzarra really turned around the 2nd half of S2 and S3 was on another level to this point. A little disturbing that they either pushed him elsewhere for monetary reasons or he felt like it was necessary. It's pretty much the biggest (non-NFL) cable show of all time. Not sure how you let that walk away.