Losing the fixed-width font is annoying to me. I frequently have code or logs in emails, and these are horrible to read in a variable width font because columns don't align, and ^^^ indicators don't line up. I'm curious why this was abandoned.
I'm pretty sure even Labs go through some sort of QA process.
However, even with fixed-width available, I almost always go after the "show original" feature* instead. I find that having the message isolated in a separate tab altogether makes it much easier to read.
* "Show original" is under the drop-down menu for the message itself.
Admittedly everyone laughs at the terminal theme. But it does the trick. And the green on black is surprisingly comfortable to read.