[sago]

This hurts when they deal with equations like

x^2 + y^2 = k

and can't even begin to understand how you could graph something like that.

It took me a while to grok it, so I second his experience of this not being well stressed (30 years ago, admittedly). I did further maths A-level (and got an A), but didn't grok it until uni.

[bitwize]

Having long been fascinated with computer graphics this bit was easy for me to get. I imagined a computer raster-scanning the plane and plotting a point when the condition in the equation was true. I just had to scale it up in my head from screen resolutions to infinite resolution and infinite extent and... oh boy, this is already starting to make my head hurt...

But the gist of it I got. And I admit, I'm unusual.