I think his hotness is directly related to the quirkiness of his character (and a good deal of that quirkiness is thanks to GDL's sly talent). That's why so many fans zoomed in on him after Cyberwoman (as unattractive as he looked sobbing constantly). Plus, that episode gave him so many interesting parallels with Jack; hello, Janto! And that's part of the character's appeal as well: he's perfect boyfriend material.