I also remembered that I hate touching my mouse, and I prefer to keep my ring finger on "page down" when reading on a computer. Having to physically lift my hand, move it to the mouse, find the cursor on the screen, aim it at "next page" or whatever then click, and if I misfire... start over. This really breaks my flow and is quite disruptive.
I know I'm not the only one who cares about stuff like this.
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Addendum: I do appreciate that "divisible by 5" quirk you mentioned :).
My quirk seems to be that I don't really consider my comments complete until I've reread and edited them something like 5-10 times. I thinks that's why it makes me kind of sad when my shortest comments (e.g. http://news.ycombinator.com/item?id=1283979 and http://news.ycombinator.com/item?id=1219118) are the ones that get the most upvotes... I spend so little time on those comments, as compared to one like http://news.ycombinator.com/item?id=1284130.