It seems to come up quite often. Someone wants the language barrier bought down. Even if it requires questing or skilling up, they want to be able to talk to the other faction. It would even make lore sense, since at the least, Undead and Blood Elves should probably know common, and Thalassian is probably close enough to Darnassian that someone who knows one language should probably be able to get the gist of the other. That said, Blizzard's held pretty fast to the principle of squelching cross-factional communication. The only way you can make yourself known to the other side is with the default emotes, or sometimes with a bit of creative typing that can only convey crude messages.
Honestly, at one point I was pretty gung-ho on removing the language barrier. As an RPer, a big part of the fun for me is being able to talk, act out scenes, say stuff in character, and all that. It was sort of annoying sometimes that I could be in an epic struggle with, say, a guild of Undead assassins, but any actual communication we made, be it OOC arranging of the storyline and in-game events or IC trash talk, would have to all be on message boards and email. It loses some of the spontaneity of in-game interaction. That said, lately I think I've decided that I'm fine with the current of level of cross-faction communication. Talking to the other side would cause more trouble than it would be worth.