My guess is that private server experience has biased your opinion as to what Vanilla WoW was, in an incorrect manner. I could be wrong, but that’s the assumption I have.
There’s an interesting post on Reddit about why there are so many bugs on private servers, where someone is responding and speaking a little to the complexity of software development. I’m reading it now, and haven’t finished yet. Here’s one of the interesting parts:
“So you have to code the entire emulator with this data once you have figured it out, encode some sort of database system to store and load everything, work out what all the flags and enums mean (there are thousands), and code it all up. To put it in perspective, the current 3.3.5a TrinityCore emulator counts at about 300,000 lines of code if you run a line counter on all C++ files. And this isn’t just simple programming: you are concurrently writing to handle thousands of connections at the same time all actively requesting and consuming content.”
Ref: https://www.reddit.com/r/wowservers/comments/31k5yi/why_are_there_so_much_bugs_on_private_servers/