MediaWiki architecture document/How and why/questions

If you had to pick, what are 5 key decisions MediaWiki's developers made that were very insightful? Why?

And if you had to pick, what are 5 key decisions MediaWiki's developers made that were, in retrospect, the wrong choice? Why?

What are MediaWiki's idiosyncrasies? What makes it special compared to other PHP software?

What would you say were the main milestones in MediaWiki's history?

What decisions and improvements improved or hurt MediaWiki's performance?

How would you describe our attitude to backwards compatibility?

Anything else you think should be included in a document about MediaWiki's architecture and history?