I guess it's a balance between over and under-engineering. The thing is, more often that not the person coming along to do 'external modifications' to that code is actually you, a few months after you wrote it.

You can adopt some good coding practices that ensure that the code isn't a spaghetti mess that's hard to understand when you come back and look at it. That's different than designing a system to be pluggable from the start.

So, I haven't watched any of Notch's livecoding videos, but for those who have, what kind of bad is the code? Is it bad "this is a mess and I won't remember what these variables do in 3 months" or is it just simply designed to handle the most immediate case (coded well but not with plugins in mind)?