The group is about 60% through level 74, with one exception-- I replaced Karianne (the BM spec'ed hunter) with Bonuss, my paladin. He joined the group at Agmar's Hammer in Dragonblight. The previous group had done quests in other parts of Dragonblight, so the paladin still had not completed the quests at Agmar's Hammer. I completed those and that got the paladin to level 75, and he is now about 40% ahead of the group. That's fine, as he was able to upgrade three gear items with level 75 crafted tank gear. Once the group completes the quests that Bonuss already completed, they'll close the level gap, but in the larger scheme of things it's not a concern.
(And yes, I have slowed down some. Questing in Northrend can be a bit 'grindy', especially now that the level increases don't come as quickly. So I do not play as much in order to keep the game from becoming a chore. If a game becomes a chore, then why are you playing it?)
It took a short while to get used to the 'tank leader' group setup. With the shaman as leader and hunters in tow, it made for very quick attacking and killing. At first I was having the paladin aggro mobs for the kill, then decided that it was easier to simply let the hunter pets continue to tank normal outdoor mobs. At this point the kill rate picked up once again. The paladin tanked an undead dragon named Sarathstra for a group quest and that worked out well. In fact, he had tanked it down before with the previous group. It's a very basic test of tanking, healing, DPS, and aggro. The dragon hits relatively hard but not very fast, and has a fair amount of health (about 265,000). I was able to test my tanking, DPS, and heal macros, and this allowed for some modifications, particularly for the healing macro.
I also killed the elite owl Alystros for this quest. Alystros has around 118,000 health and hits pretty hard and fast, but as long as you control aggro it is a straightforward kill. He does have a knockback, and thus positioning and/or quick re-positioning are useful. He can also slow your attack and casting speed by 100%, but this is no real problem for the paladin. His aggro spells are all instant cast (and thus unaffected by slows) and his DPS is only a small part of his aggro generation.
Lastly, I completed the quest Do Unto Others, which requires you to kill yet another elite NPC. High General Abbendis has around 96,000 health and better DPS than Sarathstra. But now that I had my group macros in order, she was an easy kill, although I lost track of aggro and she flattened one of the hunters before I regained control (which was entirely due to my slow reaction). These are basic tank-and-spank battles, as the NPCs don't do anything aside from single-target attacks. Well, Abbendis does a point-blank AE (consecrate) that can tick quickly for moderate damage, but this wasn't a problem with a dedicated healer and three dedicated DPS.
The paladin/hunter/shaman group has a nice synergy to it. The paladin can buff the DPS with Blessing of Might (extra attack power) and the shaman with Blessing of Wisdom (extra mana regeneration). The hunters can use Misdirection (which transfers their DPS threat to the paladin for a short time) to help with aggro control. With three hunters, I can cycle through it during a fight to help the paladin build up an enormous lead in aggro. The shaman can keep Earth Shield active on the paladin, and has an array of heals for both the paladin and the group. Riptide provides an initial heal and a heal-over-time effect, and Chain Heal can be useful for healing both the paladin and the hunter pets (and with the Chain Heal glyph, I can heal all four each time). Even in relatively poor gear, she has a pretty good spell critical percentage, and has a talent that can boost her spell crit substantially for a short time.
One possible obstacle is the lack of spell DPS. The shaman is a healer, and the hunters do primarily physical damage. They can do small amounts of spell damage, as can the paladin, but it's not enough to deal with the health that dungeon bosses have. I doubt that it will be a problem, but won't know for sure until I run into a mob that requires spell damage to defeat. If I don't, well... then it isn't an obstacle. At the moment I am not running any dungeons, but I would like to do so eventually.