There is a pretty widely accepted concept which fits around the words off....
"It's always the most ignorant one who is yelling the loudest."
I'm going to cut this whole debate down in a nut shell.
To all the people who are out there claiming that "90%" of the Heroics are "too hard" or "overtuned", there are other people out there besides TB who have posted videos proving that these instances can be cleared, often with minimal effort. If you are finding yourself not being among that group then the problem lies with you, not the game.
Just because you can do 4/12... 6/12... or even 12/12 ICC does not mean you have the ability to do Cataclysm Heroics. Furthermore, if you are failing at Heroics on such a consitant level, it is easy to put you in the following category.
You are someone who...
A. Never raided past halfass MC pugs in vanilla wow
B. Never raided past random 3.0 Kara runs
C. Never downed Illidan pre 3.0
D. Never stepped foot past the first trash pull in Sunwell
E. Wasn't around when Heroics were first introduced live
F. All the above.
Cataclysm is going to bring back to the table what should have never left, the need to actually "work" for it. You are going to have to fill all your slots with ilevel 333 gear to have the best chance. You are going to have to sit and either farm normals, farm mats, craft items, or buy said craftables to fully gear in preperation.
However, as it has been commented on several threads, Heroics will become trivalized in time, just like everything else. But this isn't going to be like WotLK. People often forget that Naxx was downed within the first 24 hours by t6.5 gear. That set the tone of Wrath, and if you are one of the people who don't fully get what that means, you are one of the people who are having trouble in the beta.