I've had many just like that. The healer and tank were focused on their jobs and had no desire to talk. They were there for Emblems, not to make new friends. (Especially since you can't friend people from other servers)

The vast majority of my LFG Tool experiences have been extremely smooth and rewarding. Heck, I'd have a new raiding guild by now if I were on the same server as some very nice people I met.

Worst thing I saw was in Forge of Souls just a couple of hours ago. Paladin kept giving everyone 10 minute Kings. I asked for 30 minute Greater Blessing of Kings, and the Paladin had NO IDEA what I was talking about. The pally also NEEDED on a caster sword with HIT RATING on it. I ignored him quickly.

I've noticed that even in silent killer groups, if someone *starts* to talk, odds are pretty good at least one of the others will reply and form a basic rapport, and the others might chime in. It's the groups where nobody takes the first step that stay silent.

I'm one of those people that tries to start at least a basic level of chatter while running a dungeon. I also find that even though I don't get myself assigned as a leader, I sometimes take up that role unofficially in the Icecrown dungeons, giving tips and basic strategies for people who haven't done the fights yet.

My greatest pug group occured this weekend, though. I did the random queue, and got into Gundrak. As soon as we buffed up, the tank pulled all the snakes in the room, then pulled the boss as soon as they were dead. Cleared the entire instance, including Eck, in 11 minutes. It was so great, we queued up for another random heroic. Then another. Then another.

Cleared Gundrak twice, Halls of Stone twice, Drak'Tharon Keep once, Azjol-Nerub once, Utgarde Pinnacle once, Trial of the Champion once, and Forge of Souls once, all with the same group and over the span of 3-4 hours. 2 Emblems of Frost and 55 Emblems of Triumph, if I recall correctly. Plus I got 3 Heroic achievements out of the deal (Good Grief, Consumption Junction, and Lodi Dodi. Just barely missed getting Less-rabi, too). Definitely not silent, but extremely deadly.

Second greatest pug group was last night. I wasn't in the best mood, but wanted to do the random heroic for the 2 Frost emblems. End up getting into Oculus. As soon as I zoned in:
"Hey, we've cleared up to the last boss, and want to try for Amber Void if you don't mind."

10 minutes later, I am up 2 Frost, 1 Triumph, and another achievement.

@DeathPaladin: Holy crap! WTB your PuG(s).

This has been my experience too :). I like to keep the chatter to a lull when running dungeons as healer.

But for some reason in every pug, when H UK comes up, it becomes this massive sh*t show. The tank pulls damn near the whole furnace area in one pull just for funsies.

I do have to admit its fun, but it's just funny that its this particular instance that every pug i'm in has the same mentality of 'let's steam roll through this entire instance kbai'. haha

Unfortunately, I've had a couple rough experiences with the "silent but deadly" types. The worst came as I was healing with my holy priest. The DPS (blood) DK insisted on making all the pulls out from under the frost specced DK tank, causing 5+ player deaths and 2 near wipes. Fortunately for him (and unfortunately for me), it was a tolerant group that seemed to just want to get through the instance without any fuss. Repeated requests of "1 sec, need mana" lead to big mass pulls. Even the 'ol "no heals for you" after bad pull only elicited a "rez plz" from the offender (the only thing he said during the dungeon). Vote to kick? Rejected. We finished the run (and I managed to somehow keep myself and the warlock alive throughout).

Sommetimes, "silent but deadly" stinks the worst.