There are many reasons to hate sticky nav bars, explained by Felix. They are annoying. They take up valuable screen space on mobile devices. They are often pointless, for example the Slate mobile one that only shows you the title of the article. I wrote a Safari extension that kills them on a view sites.
Felix mentions the real reason sticky nav is super evil: it hides content.
I often read articles by using the spacebar or page-down button to scroll. Almost all browsers implement this function, and it simply scrolls down a page. But because sticky nav is layered on top of content, instead of being separate from the page like frames used to be, it hides whatever is under it. That is not a problem when you scroll in tiny increments. It is a problem when you want to scroll a page at a time, which is much faster.
Here’s an example from Fusion.net, a site that recently relaunched and has sticky nav. I’m innocently consuming some content:
That last line ends with “they always”. Now I hit the spacebar:
Does that say “They came home the way they always of-date one”? Should I submit this to Language Log?
No, it turns out that a line is being hidden under the sticky nav. Here is the missing text:
Kill sticky nav!