The problem is that Comic Sans goes for a hand-lettered, informal look, but it's so obviously the ultra-standardized default silly font that any illusion of fun goes out the window. The detractors describe it as being stupidly clownish, but it's not clownish at all. It's more like a Botox smile, or someone who says "My friends say I'm a wild and crazy guy."
It would have been a good font if it weren't so obviously prepackaged. But not prepackaging it doesn't solve the problem, no sirree. If you ban Comic Sans, it'll just be replaced by another "hand-lettered" font - possibly a better font, but better to no avail. That font will be RUINED FOREVER by monotony and the implication of creative carelessness. The only solution would be either to prepackage ten different hand-scripted, informal, silly fonts with Windows, or to not put in any at all.
Papyrus has the same problem, but suffers from being pretty. That clash between "artistic, hand-scripted, and rough-yet-elegant" and "default font" is severe, but the intrinsic beauty of the font makes it even more distracting. It's as if I'm supposed to admire it, but can't, because it's distracting me from the content. When I see "POETRY READING" in Papyrus, I don't think, "Oh, there's a poetry reading tonight!" I think, "What? They used ALL-CAPS PAPYRUS?"
To focus on the font is to miss the problem entirely. The problem isn't fonts. It isn't overused fonts - Garamond and Times New Roman don't meet any objection from me. It's "standardized fonts that are supposed to look like individual/silly/artistic/human expressions."
On a more depressing note, when a boss uses Comic Sans to their subordinates at work, I fear that it may be the only outward face of a little, private death. This isn't the font's fault, it's just how things are.
"The office paintballing excursion's next Thursday, so don't miss it. Watch out, y'all, 'cause I'm a crack shot!"