I'm going to express what is probably a minority opinion here: I'm not a big fan of voice chat in MMOs. In fact, given the choice between typing and voice, it's a safe bet I'll always pick my keyboard over my headset - well, 95% of the time anyway.
There's no denying that some games and some gaming activities not only benefit from voice chat but, in many cases, actively require it. It's a lot easier to herd a bunch of raiding cats when you can say "ARGH! Incoming!" instead of having to laboriously type (often in semi-darkness) "Oops, sorry about this chaps, but I do think I might just have pulled a roomful of Really Bad Mob Type 374. Inco--glerk!" Anything action-based and team-based benefits from being able to speak to each other, especially since while most people can talk just fine, not everyone is a demon-speed typist.
Here's the thing, though: I am a demon-speed typist. I pretty much type for a living and I come from a generation before cell phones - or at least before cell phones were ubiquitous - and certainly from a generation before cell phones actually had reasonable typing options. I remember cell phones that weighed more than your average brick and probably weren't much smaller.
So typing is something I've been doing for a long time and I've become pretty good at it in games. I can type "Incoming!" almost as fast as I can say it, and since I don't raid it's not something I have to type all that often, at least not with the risk of drawing the wrath of 10-20 other people.
One of the things that really attracted me to MMOs was that you could talk to people from anywhere in the world and yet they were right there in front of you, at least virtually. (Yes, this is before Facebook and Twitter too.) Last millennium, however, voice chat wasn't as good as it is now - connections weren't as good as they are now. So for me, trying to play on dial-up from England on servers in the US was hard enough and fraught with disconnections; adding rudimentary voice chat to the mix pretty much made it impossible to either play or talk. Basically, it was one or the other, and gaming won.
Because even if I couldn't voice chat with people, I could still type. And boy, did we have long conversations back in those days, about anything our favorite beer (Guinness!) to politics to, you know, how to actually kill that monster that was pounding us into the dust. And because Asheron's Call didn't have all that many non-local chatting options, we used to actually hang out - virtually, anyway - in the same locations in game and just shoot the breeze. For hours.
I don't get that same feeling with voice chat, even though I understand quite well how many advantages it offers over slow and apparently impersonal typing. It's immediate. It's easier to do. It's way more in your face than typing. Which is where I start not liking it. For one thing I'm a female gamer and, though things are a little better now than they were a few years ago, the whole *gasp* "Oh wow, you're a GIRL!" thing gets really old really quickly. (No, I'm a woman, but we'll tackle that one some other day.) Secondly, I have an accent - in other words I don't speak with an American accent, and given the games I play, most of the people I play with do. So not only am I a GIRL, but I have an ACCENT!
It tends to make meeting new people on voice chat a bit of a chore, and I feel like a specimen in a Petri dish every time. The gender has to be explained - it's when a boy pixel meets a girl pixel and they like each other very much... you get the picture. Then the accent has to be explained, which in my case isn't easy because although I sound English, I'm actually not English - but we're not here for my personal history so we won't get into that.
But you know, in games, with people who might become or already are friends, you can't really just say "You know what, I don't want to talk about that. Let's just kill stuff!" Not without possibly offending people and I'm not a person who likes offending people. So I sigh and I explain as rapidly as possible and hope we can move on. Most of all I hope I won't have to deal with accent or gender comments every time I open my mouth from then on - and that's the easy stuff to deal with. We're also not going to get into the people who hit on you (based purely on the fact that you don't have a Y chromosome), the people who just can't stop making what they think are funny or flirty jokes when they really aren't, and the people who think that because you're marginally foreign and female, they have a right to find out everything you've been and done from the second you were born.
See, just thinking about me makes me cranky. And that's despite the fact that I know - at least vaguely - most of the people I might talk to on voice chat, and I know most of them are very nice people I wouldn't mind having a pint or two with in person.
Voice chat erases some of the boundaries between people who are interacting at a distance from each other, and I happen to like those boundaries, at least most of the time. That's the problem with voice: it's either on or it's off, especially for someone like me who finds it hard to just filter out the white yadda-yadda noise that makes up 95% of actual chat. When it's typed chat I can just not look at the chat box for a while, or I can change tabs, or at worst I can just shut everyone up by leaving that channel. (Mostly I make "sanity tabs" in my chat window though - a tab with nothing on but tells, or loot, or something innocuous that won't demand my attention when I'm not prepared to give it.)
With voice chat I can't do that. I'm an introvert, so the white-noise babble about what Bob ate last night or the latest cool item Joe just looted - it's tough to deal with. I've never learned to just tune people out, having been brought up to think it's rude to ignore people when they're talking, so it's all input, even if it's often inane input. And after 10 minutes or an hour, depending, I've had about enough and I just need to shut everyone the hell up.
Sorry guys. A lot of the people I talk to in games are friends and many are even good friends with whom I would happily babble all evening in person, but when there are 6-12 or more people in a voice chat all inanely blasting away, I just can't handle it. So I'll carry on typing. You know where to find me, I'm just a tell away and I type fast. And I won't hear you when you call me a sad old voice-chatless fossil.