At some point, someone on the internet decided to hate the comic sans font. Hatred can be a troublesome, difficult thing, but it is also natural enough. But finding targets for hatred can be difficult. Hating people might make them hate you back, and living in a state of stomach-churning anger at people from the South Sandwich Islands might make us doubt our status as good people.
The solution then, is to hate a font. Someone came upon this idea sometime in the last decade, and it was such a nice solution to the hatred problem, that it quickly spread to the point where hatred of comic sans has reached a point of our tacit cultural heritage, a type of hundredth monkey phenomenon. It also gives one cachet to have a strong opinion about such a seemingly obscure topic. People in flyover country are too busy eating grilled cheese sandwiches and watching reality television to have the type of virulent opinions on fonts that those of us who lunch daily with Brad Fitzpatrick do.
Comic Sans is a font that gets its name from the fact that it looks like it could be used in comic strips, and it is sans serif. It has rounded letters. The font is meant to be casual, used for "fun" messages and announcements. It would probably be quite draining to say, read War and Peace in Comic Sans. But then, no one is asking anyone to do so.
Comic Sans is one font amongst many, and it is probably well suited for some purposes. It is probably very annoying when overused, but then, most things are.
But there is nothing I can see about Comic Sans that would lead to its wide geek hatedom. In fact, if not exposed early and often to the subcultural fervor against it, I don't think many people would hold an opinion on Comic Sans. And of those who did, and decided that it was somehow unpleasant, I doubt many would decide to make an emotional issue out of it.
The internet's reaction to Comic Sans is a meme in the original sense: an idea that is passed from person to person and spreads, quite outside and behind the original context of the idea and the magnitude that it warrants.
P.S.: After I wrote this, I went and looked, and I do admit, the "m" does look pretty ugly and crooked.