Hey Sarah...I think many people feel it is an inappropriately and overused font. It was used a lot in comic books but then spread to other more serious, professional documents. Many people feel as though content written in CS isn't authored with authority and lacks credibility due to it's "silly" appearance. I have to admit that I feel it has outstayed its welcome, but I am no authority on typeface! I'm loyal to TNR, Helvetica and if I'm feeling dangerous, Garamond!
Actually, my tongue-in-cheek comment aside, I personally like Comic Sans a lot. The backlash seems to stem from a feeling that Comic Sans is "childish" because it was designed for comic books (which I'm not completely sure is true, but that's what they say).
Despite its detractors, Comic Sans remains an excellent typeface. Very readable, and in fact one of the MOST readable for people with dyslexia. A new font, Dyslexie, has been specially designed to make reading easier for anyone with dyslexia. It's very similar to Comic Sans but isn't burdened with the same prejudices. Here is a nice video about it (hope the link works): Dyslexie is a typeface for dyslectics. - YouTube
This topic came up the other day and the response to it seems so overblown. It's a font people. It's a sort of bubbly font, I'll grant you, but just a font. I get that it may not seem the most professional, but I never imagined it would spawn so much vitriol. Google it once. You may be surprised, too.