I'm somewhat disappointed that I couldn't think up more for this. I used to have a long list of things to complain about at any given moment, and some were internet related stuff. Guess I mellowed out some in the last few years.
I'm not sure I consider the whole poor grammer thing a valid answer to this since it's not exactly a habit and is hardly limited to the internet. But... it still jars me whenever I see people get possessiveness wrong on words, or use then / than improperly. Or your / you're. Or their / there / they're. Possesiveness on its / it's is a bit less of a problem to me since it goes against the common rules of English.
Anyway... not sure that I should continue the MLP discussion considering how many mines there are in that minefield (and the effects of their previous detonations), but I can completely understand where everyone is coming from there. I'm a brony, but I used to hate it whenever I had anime shoved in my face at every turn on the internet. I still don't care for anime, but I used to detest it because I couldn't get away from it, and people made it out as if it was just absolutely magically more fantabulous than western animation because Japan. Or something like that. I've learned to mostly just not care now, because I realized it was pretty stupid to get so irritated at nothing.
There are pretty strong parallels to the current brony community. Ponies are everywhere and if you don't like it, it's pretty hard to avoid ponies. It's one of the main reasons I don't have a pony avatar, for example. I know how the people who don't like ponies feel.
To be fair though, I mellowed out about the anime thing before I became a brony.