Follow breaking in Wrath...

[Tanntyn]

This was probably mentioned somewhere else, but I stumbled on a review of Wrath that contained a couple of things that could answer what is going on with the follow thing. For those too lazy to read the whole review, here is the relavent section:

Another way Blizzard found to keep things interesting is what they call "phasing" technology. One of the long-standing complaints about the MMO genre is that there is very little permanence to a player's actions. When Player A rescues a princess from an evil wizard's tower, he's very shortly going to turn around and see her back in the tower, waiting for the next player to do the same quest. Either that, or when Player B comes along, the princess is already saved, and he missed out on that content. Blizzard's solution was to implement different "phases" of an otherwise static zone. You'll start out in the beginning phase, during which, for example, a town is under attack. Completing a quest to fight off the attack bumps you into the next phase, where the town is safe and its fighting force is going on the offensive. Now, a player who hadn't done that quest could come from the same place as you, and stand where you're standing, but you and he would see two different things. His town still needs saving, yours doesn't.

So in a nutshell, the server is probably pausing long enough to check the status of the toons entering the zone to determine what phase to display and that is what is breaking follow.
I'm no programmer, nor very technical, but that is my take on it.

[Kedash00]

i'm thinking the same thing, i was flying though icecrown last night for the first time and i went into a phasing zone and all my boys stopped follow, both when i entered and exited it, i couldn't turn around and see them either they were in like the fog in borean tundra... kinda pissed me off when i came back from smoking and found my pally at the argent dawn place and my shammys still in hte middle of the map.

[TheBigBB]

Icecrown is the coolest zone in all of WoW, but it's a special zone which requires some complicated stuff to be going on serverside, which leads to characters losing follow at certain points. If you do the quest chain for Icecrown, you'll see that the zone really changes based on your progress, with an entire new town and flight point being set up after about 20 quests. Unfortunately, you need to do a couple of bad collections along the way, but after that it's a motherload of super-easy exp and great quests.

[Kaynin]

I already know most, if not all locations where I will loose follow due to this invisible 'instancing'. I've memorized them, when I fly through the edge, I just press move forward on all chars, they get in the same zone, follow key, continue.

It isn't an issue, really.