[was_a_dev]

I read most papers on a secondary 19" monitor that's mounted portrait. Seems to cover all the requirements

[Someone]

Not all ones not mentioned in this thread yet. A secondary 19" monitor ties you to a desk. A tablet would allow reading wherever people typically read paper printouts (in the train, while sitting on a couch, etc.)

[bluGill]

When I have a complex paper to read I will spread it out on the floor. A single page at a time is too limiting, I want to be able to see page 5 and page 12 at the same time.

[was_a_dev]

You're not wrong. But in those scenarios, I tend to be off the clock

[bee_rider]

It was nice to be able to jot things down in the margins (math, not just notes). Provide yourself the couple steps the author skipped (to show how clever they are).