There's a post at Kill Ten Rats that mocks the somewhat absurd nature of pulling groups of enemies in an MMO. Basically, no matter how you pull, any other enemies outside a set distance, whether they be in the same room or not, will completely ignore you, even if you pull with a honking great big flashy exploding fireball of doom.
But one of the comments (the first one, by NBarnes) puts forth an interesting idea: what if, upon pulling the first group, the entire instance was alerted to your party's presence, and began to make its way toward you?
If done right, this could make an instance really exciting, as you'd be racing against the tide, desperately trying to reach an objective before becoming overwhelmed. If there were multiple paths through an instance, you could perhaps employ some tricks to divert enemies away from you, although you'd then have them racing to catch up behind you, adding to the pressure.
This sort of mechanic is already in the game to some degree in instances like Violet Hold, or the Thorim fight in Ulduar: waves of enemies come at you and you have to fend them off. The problem here is that the location is more or less static: you basically stand in the same room and take what's thrown at you. Culling Of Stratholme and Pit Of Saron, on the other hand, have a "gauntlet" section, where enemies keep on coming at you until you get all the way through it. It's still a relatively small area of the zone, but the important difference is you have to make your way from point A to point B whilst fighting off enemies (or in the PoS case, rounding them up then fighting them all at once).
Boss fights in such a place could differ from the norm in that you'd be quite pushed for time - perhaps the party would enter the boss's room and the doors would lock for a time, keeping the approaching enemies at bay. You'd then have a clearly defined (thus controllable, from an encounter mechanics point of view) area in which to do battle. Defeating the boss could then throw up some kind of permanent obstacle to the enemies, and you'd have to leave via a newly uncovered (and also enemy-free, for now) route.
As with the boss encounters, there could be sections where you're allowed a brief reprieve, restore mana, top people off, then away you go again, into the fray!
It's obviously impossible to say how well this kind of dungeon would work in practice, and how hard it would be to balance. I also think it should be reserved for the higher levels, since by then tanks would have more experience (barring those who levelled as DPS then respecced once they reached the level cap) and would be better able to cope with the constant pressure to move forward, better able to round up enemies whilst on the move.
Really, there are many possibilities for this kind of encounter, and whether it's fun or not really depends on how well it's balanced, or the timing of enemies. Perhaps they wouldn't come in "waves" as such, but instead more randomly, with different mixtures: at one point you might encounter a large, tough monster on its own, other times weaker but more numerous ones grouped together.
The element of randomness would, I think, add to the experience, as otherwise you'd get into a situation where people could simply learn the sequence and dance their way through it, eventually narrowing it down to one optimum path. I've read in various places that at least some people would prefer more dynamic encounters (unless they're PvP-like, where the rules are completely different), and I believe this sort of idea could work well and allevieate some of the grind if it offered enough variety.