Maybe cognitive friction in our reading experiences -- be it in typeface choice or annotation mechanism -- is a good thing.
One of the really tough questions to answer in relation to any technology is: When do you make something easy and when do you make it hard? This problem is perhaps most obvious in the realm of game design, since people get bored by games that are too easy and get frustrated by games that are too hard. So game-makers have to learn to split the difference, which in practice means alternating between the easy and the hard. You allow gamers to get some momentum and confidence by completing easy tasks, which helps them to push through the annoyance and even anger that can arise when a nearly intractable challenge comes their way.
But this problem occurs in other technological arenas too. Consider typography, of all things. In his recent book Thinking, Fast and Slow -- which is fascinating in more ways than I can tell you right now -- Daniel Kahneman explains research that has been done on the cognitive burdens placed on us by various type designs. A well-designed text, with a highly legible typeface and appropriate spacing, places a considerably lighter cognitive burden on us than a badly designed page. It works in conjunction with other factors, of course -- but it matters:
A sentence that is printed in a clear font, or has been repeated, or has been primed, will be fluently processed with cognitive ease. Hearing a speaker when you are in a good mood, or even when you have a pencil stuck crosswise in your mouth to make you "smile," also induces cognitive ease. Conversely, you experience cognitive strain when you read instructions in a poor font, or in faint colors, or worded in complicated language, or when you are in a bad mood, and even when you frown.
Reading a page done right is like sliding on the ice: we just flow right along. Take a look at this smart post by Dan Cohen on how much we value cognitive ease when reading, and how many recent tools provide it for us.