The first 5-man instance you can do in The Burning Crusade is in a place called the Hellfire Citadel. There are two: Ramparts and Blood Furnace. Ramparts is the lower leveled of the two - I think 60-62.
I think casual players will be very happy, it is a short instance that is easy to get to. I think hardcore players will run through this once to see what its about, get their quest credit and move on.
I did it with a pug, using the lfg function - what is great is once you form a group you can use the meeting stone to summon them - very, very nice feature for groups without warlocks. How many times have you sat around waiting for someone to travel to Dire Maul? ZZzzzzzzz.
We had a nice class mix at first - druid, priest, warrior, mage and warlock. The warrior disconnected early on and didn't come back, yet we still were able to pick up a paladin and continue on. I'm happy to report that both I and the druid were not healbot specced and the druid tanked and I was able to heal just fine. Given the fact we waited a little to get a replacement, I'd say it only took us an hour and a half or so. We wiped once because the mage didn't decurse - but the graveyard isn't very far and the instance itself is short in length and easy to navigate.
Like I said, casuals will be happy with this but if it was a group of people from my guild in the gear we currently have? We could probably do it under an hour. And after doing it once we could probably shave it down to 30-45 minutes. Maybe even quicker because it wasn't as hard as a baron run.
There was nothing there anyone really wanted. We greeded on all the green and I sharded all the blue BoEs. But I can see some casual players coming away with some upgrades.
I think it is a nice introduction to the new age of "Casual World of Warcraft".
6 hours ago