First of all, I don't support every change that Blizz makes and I hate some of them but there are a lot of people that is just plain unpolite and talk like they know how everything should be done. I'll only talk about what I know for certain, and I would like to have a friendly talk with people that really understand what happened here, and also learn from you guys, this will be long so I would understand a TL DR .
I see a lot of people talking about developers like they know everything about it, i'm a programmer and I certainly know that a bug, even a tiny one in some stupid 2000 line code can break havoc, now, how can we compare this to what happened in this prepatch. First take into account that this game has probably more than 4 millions of lines of coding. They have (i'm speculating here) code working below code and that code below that other one and a lot of those connected, they probably try to add more and connect everything every time the add something new or change something. Now, a lot of programmers and developers have changed over the course of the years in wow development and of course new ones have to adapt to everything they find and that takes time.
In this prepatch they changed not manually like they said they did in WOD, but with some tools they created some stuff that is working with old coding and systems. This "Tool" will be awesome for them once it's made better but it needs testing to see at what level it can affect the game changing old stuff with it, and let's be real, the mayority of the people in the PTR are there just to see the changes in their current main and not to test everything, the best time to do those tests and it's a hard pill to swallow, it's in live.
I did expect that these changes would break random stuff around the world, quite a lot of code changing and it's impossible to test every possibility, I have seen and I have laughed even to myself how sometimes you test everything (taking my case for example) in a application I made for 5 people where I work, seriusly guys, I fucking tested everything and made some of my co-workers try to find bugs and everything was perfect. But then, there comes someone to use it and manage to find something, it happens. What i'm trying to say, is that if it happens with a simple application and only 1 developing I can't imagine what can happen with millions of lines and dozens of people.
P.D: really sorry if my english is impossible to understand and would completely understand a TLR, if you have doubs and do not understand something I said, i'll try to explain myself to the best of my ability. Cheers!