Aside from the opinions, which is quite a bit.
Polish takes time.This avenue of thought really needs to die. You wouldnt believe how many people think that a bad launch is okay because "wow's launch was bad too!" or a bad system is okay because "vanilla wow's systems were even worse!". I've heard it a million times and its proven wrong again and again. Games should be releasing with at LEAST as much polish as WoW has currently, because they have examples to work from, advances in technology, and a better base of code to work from. WoW is extremely limited because the code is so old they actually have technical difficulties making some changes. New games should not have these problems.
Want to know the best way to find bugs? And this is very very true in every single software. Release it to the public. Nothing beats thousands of users using the product to find the bugs that you overlook.
Some bugs are caused by time, some bugs are caused by very small but repeatable instances. Some are hardware or software conflicts.
There is no examples to work from is the thing.
WoW's source code isn't open source. You can't get examples of code that are closed. You can get a general idea of how something works, and may or may not be close to it, but you won't be able to know how it all works.
There is no such thing as 'better base of code'. There's engine, which better is debatable on the usage. Some engines just aren't fit to run certain types of games. Some engines are great for certain things. This however doesn't make any of em better. (Presuming that they're not full of bugs that is).
Programming isn't as simple as A + B = C. There are so many variables to take into account. You can make a program unoptimized but works, but issue obvious is it's unoptimized. However once you go into the land of optimization. It gets a hell of a lot more hectic. These things actually take time.