So far with all the new class changes I've had a chance to run or group with some of all the classes though not all. I can't say I've being in a group with a Shadow Priest or a DPS Warrior usually. But All my groups are PuGs. I'm not apart of any guild in Northrend just liking to go it alone and do other things as i feel so instancing are all random groups and invites.
Most the classes from that are in tBC I'm pretty much used to. Some classes more than others in a 5 man group as a Protection just works much better with some classes than others. Its a bit different seeing a Shaman with wolfs along in a fight and wondering where they came from as the tank, as bit different that is. But most classes seem the usual. Rets seem to work well in a groups that I'm in but that's still hard to tell when you can be busy tanking and learning Dungeons and mob tactics.
Biggest class so far getting used to is probably with Death Knights because they are new and experiences are new with the class. Some Dungeons I've been in groups with 2 Rets for dps or one group I had 3 Death Knights a Hunter and a Priest. That's a bit different there with multiple of one class in a group of 5. Working with a good Death Knight seems fun with at least one DK in a 5 man group based on how my Paladin AoE works tanking.
Its has being sometime of a challenge sometimes AV Shield pulling a group of mobs then start heading towards the mob closing the distance only to have some DK Death Grip it from right where the mob was before I got to it and then having to run back to where I was. As the tank that has more than often being a bit annoying to deal with. Sometime I think of it ahead of time marked a Caster in a group, AV Shield the Caster and have the DK yank the mob from where it was pulling the rest of the group that was shielded. However, I still find this a bit annoying or annoying to deal with in however I decide to pull a group in more than one ways.
However I'm still getting used to being in groups with DK's as they do add a bit to a group as well as with buffs. Two DK in a group is bit much to deal with and three DK's though can seem like fun, to me is more of a challenge to deal with with all the uncontrolled AoE being tossed or spammed around in the groups that I've been in and sometime without regard to the target mob. One good DK in a group seems to work well with a balance of other classes in groups that I've tanked with beyond that its one DK to many for me as a usual 5 man group.
Their will be allot of DK's in WotLK and few will be really good at first learning the class or being as experienced as the other more well known classes. Many will have to learn to work with other classes in how they play. With so many new DK's can only hope they don't end up with some stigma like Hunters have in WoW learning to group and work with other classes.