Currently, <!-- hidden HTML comments --> are displayed as a small gray circle: (!) The full text is available on hover (for desktop users). Should we instead display something like (!) hidden HTM (the first few characters/not the entire comment)?
I decided to play with this a bit, as can be seen in the patch...
This is with the comment preview being truncated to 20 characters.
There's also the somewhat visually excessive look of faking some comment markup:
Would be interesting to see on some wiki articles (e.g. Obama). I think it would be styled like the invisible templates, so we can omit the <!-- (a non-wikitext user shouldn't be familiar with that syntax anyway).
If we see a rash of deleted comments in edits on comment-heavy pages, we could perhaps invest in some sort of UX for comments (and invisible templates, which presumably suffer from the same issue) to drive home that they're invisible in the final output. Add some explanatory text to the context item, perhaps?
That said, I don't know if the label style is any more at risk from a rash of deletions than the former just-an-icon style.