- More #s mean a larger visual footprint, which would be more appropriate for higher-level headings (cf. larger font size in rich-text rendering, and of course the = and - underlines). This is what bothers me the most.
- I tend to write documents bottom-up, starting with lowest-level headings, and higher-level headings are only introduced as the document evolves. Because I don’t know how many levels I’ll end up with, starting with a single # for the lowest level would work better.
- Lower-level headings are more frequent than higher-level headings, so having to type less for them makes sense, as well as reducing the likelihood to get the # count wrong (because you usually know what is a bottom-level heading, or the next one up, and you don’t have to know how many levels there are overall).
The existing convention mirrors the numbering levels of numbered headings, but I don’t find that visually convincing. It also matches the HTML h1-h6 elements of course, but nowadays that should be less of a concern.
Another thought is that maybe the headings hierarchy should be unlimited in both directions, so that you are never forced to shift all headings. One possibility to achieve that could be:
### even higher level
## higher level
# base level
.# sub level
.## sub-sub level
.### sub-sub-sub level