Earlier quoted context omitted.
SPJ about Comic Sans: > This is a very funny question, "Why use Comic Sans?" So, all my talks use Comic Sans and I frequently see remarks like 'Simon Peyton-Jones, great talk about Haskell but why did he use Comic Sans?' but nobody's ever been able to tell me what is wrong with it. It's a nice legible font, I like it. So until somebody explains to me ... Ah, I understand that it's meant to be a bit naff, but I don't…
Because it's difficult to read.
(I'm prepared to believe you, but the main complaint I've ever heard is that it's "silly" or "frivolous" or... well, actually I haven't heard much beyond that.)