I run a mixed group: Prot Pally, Ele Shaman, Boomkin, Fire Mage and Holy Priest (might switch my mage to Arcane and my priest to Disc though, have to do some research on that).

After hitting 80 and running some heroics I can see the appeal of a Pally/DK and 4 shaman group. This setup allows you to have an extra DPS (i.e., no dedicated healer) but still have AOE heals. Shaman seem to be easier to gear up to get some serious spellpower as well. It's pretty much a killer group. I'd even guess it's really the best multiboxing group for PVE (although I have no first hand experience).

In my case, I wanted mixed classes so I could have a lot of options later on if I switched to raiding...something I am not sure I could do as a boxer. A mixed group is typically just a tank, 3 caster dps and a dedicated healer. In general, the dps classes make no difference whatsoever. You will work out the macros. All I do is hit a single key. I would do the same if they were all shamans, warlocks, mages or a mixed bag. The big difference is a dedicated healer vs. four DPS that are mediocre [but satisfactory] healers.

To maximize your DPS the synergy is important with mixed classes. For example, having both a warlock and a boomkin might not be as good as having just one and a totally different class due to key buff stacking issues. Just pick any 3 DPS classes you like but make sure their key group buffs can stack.

As for a healer, a good AOE healer is important IMO. At least for PVE. Anything is doable of course, but being able to spam some AOE heals is much easier in a lot of cases.