While I don't necessarily agree with Keen in my previous post about class consolidation necessarily decreasing combat depth, I do agree with him insofar as having support classes can be fun.
When folks talk about the Holy Trinity, it's basically a way of dealing with threat mechanics in a way that makes some sense to the players. Tanks deal with taking damage, Healers fix damage taken, and Damagers (or DPS in more common parlance) take out the baddies.
It wasn't always like this, though. In older MMOs like Everquest, you also had Buffers and Debuffers. While WoW has rolled those two class archetypes into tanks, healers, and DPS, I find I miss having that very specific role where your job couldn't necessarily be numerically evaluated quite so easily.
When you look at D&D 4th Edition, they don't have a trinity, they have a quartet: Leaders (healers/buffers), Defenders (tanks), Strikers (DPS), and Controllers (debuffers). I often find playing a controller or leader is actually a lot of fun. The ability to move people around combat, change the face of the battlefield, or give folks bonuses to their damage or to hit, is harder to measure than damage taken/healed/done, but is still an immense help to the party. To be honest, I prefer one of those roles over Strikers, because while big numbers can be fun, I find my combat options tend to be more limited as a Striker.
In Vanilla, WoW did effectively have buff classes in the form of early Paladin and Shaman. Though, I'll grant you that having to spend all your time buffing 40 players one-by-one with Kings, then starting over and buffing them again one-by-one because it would expire soon wasn't a good implementation of a buff class. However, with EQ-style Bards you could "twist" songs to provide up to two buffs at once to your party, and you had a number of different buffs to hand out based on the context of the situation.
Now, Blizzard did basically decide that buff classes as they were in Vanilla were not the way to go. Over time, every class was relegated to DPS, Heals, or Tank, and buffs and debuffs got distributed across all classes. Bring the player, not the class, right? Though you still need to bring a specific role.
It makes me wonder if rolling up buffs and debuffs into the Holy Trinity was required, or if there's room for classes specifically devoted to helping other people? Or was it perhaps a side-effect of a very numbers-driven sub-culture, where Recount was king, and if you weren't pulling numbers, you weren't effective? In WoW, damage meters of some sort existed as far back as late Vanilla at least.
The ability to measure your output is interesting and mildly contentious. It's very difficult to improve if you don't know how much damage or healing you're outputting or taking. On the other hand, if there was no ability to measure that number, people wouldn't focus on it either. RIFT tried this for a while, but the developers eventually reneged based on player feedback, so I'm not sure that particular genie can be stuffed back in the bottle.
But as per my point earlier about D&D 4th Edition Strikers, perhaps that's the more important point. Strikers I find dull because they exist to do damage and nothing else. Perhaps rolling up buffing and debuffing into DPS doesn't have to be about buffs and debuffs being dull. Perhaps it could be that just plain old DPS is dull, and giving them buffs and debuffs to handle as well as DPS makes the role more interesting?
While I don't think we'll ever be able to see a buff/debuff class in WoW--it just doesn't fit in the playstyle anymore with the distribution across classes and rolls having already occurred--I think perhaps we might be able to see something like that in other games that don't quite have the Trinity so entrenched.