Unfortunately the way that comments are rendered depends on which view you use. There's probably a subset that works for both but many [new.]reddit.com users won't realize that their post doesn't render correctly for old.reddit.com.
Given this design, I suppose one must suffer: [new.]reddit.com adoption or the experience for old.reddit.com.
The one that keeps coming up is how 'code' is rendered. Old reddit uses four leading spaces for a code block and IIRC new reddit uses triple-backtick. It's probably slightly different, not-quite-100% compatible markdown dialects.
Admittedly it's a problem more common on the more technical subreddits.
Ah. I have noticed that one actually, but I chalked it up to someone confusing Github/SO/Reddit markdown. Didn't realize that Reddit itself was inconsistent.
Given this design, I suppose one must suffer: [new.]reddit.com adoption or the experience for old.reddit.com.