"Alternatively, gray out the button more based on how a comment ranks (higher = harder to see)"

I suspect you were joking, but I don't think this will work because everyone knows where the upvote button is.

This did however lead me to the idea of hiding the upvote button in more difficult to find places the higher upvoted a comment is. And now I'm laughing about this when taken to a surreal conclusion so thanks! :)

(e.g. "This top comment is brilliant, where's the upvote button? How did HN get it inside my fishtank?!" etc.)

Someone else also suggested disabling upvotes for comment no. 1. I replied here: https://news.ycombinator.com/item?id=5430753

I'd guess people would eventually find the button anyway, even if it's grayed out? I mean, they know where it's located, and notice that they can click on various shades of gray... I'd guess they'd feel somewhat upset about the odd UI, with clickable "disabled" buttons :-)

graying would serve to make it slightly more annoying to see/click -- to dissuade casual upvotes.

You could also make the hitbox of the button increasingly smaller - to where the no.2 comment has a 1x1 px box.

tl:dr make it frustrating to upvote top comments -- if they really 'deserve it' people will go out of their way to do it