Some comparisons and analysis:
When the font size is small, and the ‘bold’ text has a much heavier weight than the regular text (left-hand version), the eye is drawn to the bold text. This is both because (a) reading the regular text is effortful (due to the small size) and the bold stands out and thus requires greatly reduced effort, and (b) because of the great contrast between the two weights.
But when the font size is larger, and the ‘bold’ text is not so much heavier in weight than the regular text (right-hand version), then the eye does not slide off the regular text, though the emphasized lines retains emphasis. This means that emphasis via bolding does not seriously impact whether a reader will read the full text.
Not much to say here, except that how different the italic variant of a font is from the roman variant is critical to how well italicizing works for the purpose of emphasis. It tends to be the case that sans-serif fonts (such as Freight Sans Pro, the font currently used for comments and UI elements on LW) have less distinctive italic variants than serif fonts (such as Charter, the font used in the right-hand part of the image above)—though there are some sans-serif fonts which are exceptions.
Appropriate typography is one way to increase a post’s navigability/skimmability. A table of contents (perhaps an auto-generated one—see image) is another. (Note that the example post in this image has its own table of contents at the beginning, provided by Raemon, though few other posts do.)
This is a perfect case study of points (1) and (2) above. Warnock Pro (the font you see in the left-hand part of the image above) has a very distinctive italic variant; it’s hard to miss, and works very well for emphasis. Charter (the font you see in the right-hand part of the image) has a somewhat less distinctive italic variant (though still more distinctive than the italic variants of most sans-serif fonts).
Meanwhile, the weight of Warnock Pro used for ‘bold’ text on the left is fairly heavy compared to the regular text weight. That makes the bolding work very well for emphasis, but can also generate the “people only read the bold text” effect. On the other hand, the bold weight of Charter is distinctive, but not distractingly so.
Finally, as in point (1), the larger the font size, the less distracting bold type is.
Here, for reference, is a brief list of reasonably readable sans-serif fonts with not-too-heavy boldface and a fairly distinctive italic variant (so as to be suitable for use as a comments text font, in accordance with the desiderata suggested in my previous comment):
Frutiger Next *
IBM Plex Sans
Optima nova *
(Fonts marked with an asterisk are those I personally am partial to.)
Edit: Added links to screenshots.
One thing that’s worth noting here is there’s an actual difference of preference between me and (apparently a few, perhaps most) others.
When I use bold, I’m specifically optimizing for skimmability because I think it’s important to reference a lot of concepts at once, and I’m not that worried about people reading every word. (I take on the responsibility of making sure that the parts that are most important not to miss are bolded, and the non-bold stuff is providing clarity and details for people who want them)
So, for my purposes I actually prefer bold that stands out well enough that my eyes easily can see it at a glance.