On the desktop site, most of the vertical whitespace in one-line comments is to make space for the flag button, which is below the vote button. I find this whitespace to be excessive, especially given that my monitor is wider than it is tall.
I propose moving the flag button next to the vote button, instead of below it. For example (current on left, proposed on right):
In example 1, after the change, the entire set of non-hidden comments fits nicely in my browser window.
I think this improves readability of comment threads by grouping the comments slightly closer together, providing a relatively consistent amount of white-space between single-line and multi-line comments, and reducing the amount of white-space (letting you see more comments at once). I would like to see this done on all of the sites.
In the above examples I just moved the element but did not adjust spacing. Some small amount of extra horizontal spacing between the buttons could help separate them a bit.
I have also created a user-script that can do this on the sites if you want to see what it looks like (sorry if it's ugly, it was a bit of a "hello world" for me; I know it works on Chrome at least). The script does add spacing between the buttons (a 0.75ex padding looks nice).
Here is an example with a slightly wider spacing between the buttons (0.75ex). They're about the same distance apart as the current vertical orientation: