It's about the fact that, no matter what, our fighter can blow Come and Get it, and dictate the movement of my monsters. It's about the fact that, in a big fight, my players can basically stun-lock my big bad monsters, and if I say "no, that doesn't happen" I am breaking the rules and depriving them of their core strengths. It's about the fact that, if I want to do something, my players feel they have the right to say "no, this doesn't happen."
Well, keep in mind that Come and Get It was always an outlier in working that way - and that is now no longer the case.
Still, the problem remains in the form of daily powers where stuff happens on a Miss or via an Effect.
The answer, in my opinion, is to absolutely say "that doesn't happen". Not on the spot, of course - but more monsters should feature immunities. I'm in a Paragon game where our rogue inflicts crippling penalties on enemies that make it almost impossible for them to hit - as long as they are vulnerable to fear. Yet we are fighting primarily undead. It feels strange that almost none of them are immune to fear.
Im not saying everything should be protected from the PCs strongest attacks. But important monsters
should have defenses against stun - either immunity or by reducing the effect or otherwise. Appropriate monsters can't be dominated or forced moved or whatever.
In my epic game, I tended to give such benefits to important enemies - though usually with some downside. A raging elemental golem might not be able to be stunned - but would instead, for that duration, become vulnerable to damage. The PC sees a clear result from their power, without it ending the encounter outright.
I also tried to set a tone of "feel free to use whatever tricks you want... but expect to be met in kind." So if the PCs really trick out to stun-lock enemies, they can prepare to meet lots of enemies who do the same. No one really wanted that, so stuns were used relatively sparingly.
I think part of the problem is that some conditions are handed out more freely than they should be. And that some powers are written so that nothing the monster can do can prevent the outcome from happening. Which is a shame, because it does get back to the system mastery issue of third edition - instead of the DM needing to figure out how to protect the key monster from SoD, he needs to figure out how to protect them from being stunned, or from the sorcerer sticking them in a teleport loop for absurd amounts of damage.
In some ways, while the effects are weaker, that almost makes it more insulting - your epic monster is being rendered inept since he's cornered by a defender, and if he tries to move away, gets hit by an OA that knocks him down and trips him, and then he gets dazed by the rogue and can either spend the next round flailing on the ground, or waste his turn standing up... etc.
In the end, it mostly worked out - our Epic game remained enjoyable. PCs could dominate some encounters, but would run into occasional enemies that really shook them up. That felt appropriate epic, overall. But it does require some attention to what encounters you present, and how you present them, and I can see the frustrations that can come with dealing with that. (Especially if you try diving right into Epic level out of the blue.)