In my newspaper days we stuck to around 60 characters as an optimal line length for readability. I've seen up to 80, but even that seems to be pushing it. Once you stretch out the lines so much it's hard to track back and forth from the end of one line to the beginning of the next line.
I'm reading the parent, top-level comment on a Macbook Air with a 13 inch screen and the first line is a whopping 194 characters long. Reading anything of length on this screen is decidedly uncomfortable when browsing HN.
I agree that simplicity is a noble and useful goal, but when it comes at the expense of usability it's hard to swallow.