Anyways, what you've written here is good, and definitely encompasses some things I thought about. Two notes for you:
1. I explicitly preferred Arial and Helvetica over the generic sans-serif is because I found some other popular web safe fonts didn't look nearly as good, for example, Open Sans, mainly due to the large x-height.
2. I don't think rem incurs much cognitive overhead over px, and the main reason is that it scales with the user-adjustable font size. Try changing your browser's font size from 16 to say, 20. You'll notice that with px max-width, # chars per line will decrease a lot, affecting readability. in contrast, rem max-width will scale nicely.