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.
The thing that most contributed to changing my mind was the arenas. When the Arenas first came out, there was no language barrier for same faction teams. Now, even if we're fighting each other in the arena, you'd think maybe there would be some same faction pride, we could play fair be nice to each other, and all that.
No, sorry. Alliance are jerks, apparently. Admittedly, the incidents were few and far between, but they were there. My brother and I would queue up for our 2v2 matches. We'd get alliance teams. Sometimes they'd start with the taunts the moment the gates opened, boldly saying they were going to play with us. They might kill one of us and toy with the other, running around, chain-CCing, letting us DPS them for a while then healing up, all the while with the yells and taunts. Honestly, if that's how people are going to act, I'm suddenly seeing the wisdom of limiting cross-faction communication. To me, it's not too much to expect a little bit of sportsmanship and a sense of fair play, but if there's too many people who just want to be jerks, it's probably for the best that there's a few extra barriers between them and easy jerkdom.
These days, they've squelched communication in arenas as well. Now when someone from the other team says something, even if we're both Alliance, all I see is "PlayerX yells/says something to his team." Ever since then, I don't think we've had one team play with us. To me, that's a good sign that limiting cross faction communication is, in fact, an effective deterrent to griefing. Apparently people aren't quite as keen on being jerks to you if they can't taunt you while they do it.
While it might be nice to yell speeches back and forth with your evil Blood Elf archenemy when roleplaying, or even just set up a Booty Bay AH deal without message boards or alts coming into the equation, it seems the lure of being a jerk is probably just too much for some people to handle. So to me, it's probably for the best that the language barrier remain in place.
I suppose if you're really dying for trash talk (And you are either on a PvE server or have two accounts), you can just log on over to the other side after the battle's over. At the least, the process of logging out and logging back in might give you time to cool off.