YouArentGonnaNeedIt (often abbreviated YAGNI, or YagNi on this wiki) is an ExtremeProgramming practice which states: "Always implement things when you actually need them, never when you just foresee that you need them."

memory usage and (lack of) parallelism are concerns

I really hope they keep breaking it. Being the lead on a library for several years, most of the forced refactors were pretty straight forward and in almost every case made our code either more sound or easier to be consumed. Now I work on a runtime that embeds TypeScript and 3.5.1 has broken some code, thought it took me all of about 15 minutes to make the changes to adopt it, and in every case, it broke because we were being a bit loose with the types. While it didn't find any bugs, it made the code more "safe".

When defining accessors in Ruby, there can be a tension between brevity (which we all love) and best practice.

Setting an instance variable by going through a setter is good practice, and using two access modifiers is the way to accomplish that for a read-only instance variable

Each of the programming language generations aims to provide a higher level of abstraction of the internal computer hardware details, making the language more programmer-friendly, powerful, and versatile.