If one looks at it that way, then the only cards that could be broken are combo components and the mana acceleration/card drawing that gets them.
Not all decks are combo decks. In fact, most of them aren't. I would say that any cards that create huge game swings, even without being part of a first turn kill, are still potentially broken...
By itself, Earthcraft turns all your creatures into Llanowar Elves, or a little bit better even...
It needs to be combined with other cards, typically local enchantments like Overgrowth (which are bad by themselves) and the combo was in the past three cards, not counting a land.
The Squirrel Nest combo cuts down on that, but it's still a two card combo and neither of the components does anything by themselves, mainly.
Also it needs another turn in order to make the attack. Unless some sort of weird haste card were able to be brought in, the deck would have a fundamental turn of two, which isn't bad.
A two card infinite combo is no lightweight, after all...
But it's not as good as existing combo decks like The Perfect Storm...
I would definitely call Earthcraft broken, since its effect is still quite powerful.
To me, Black Vise is more broken. It costs only one mana, and does not need a three mana local enchantment to be good. It can deal damage by itself against most decks, more often than not, that damage is greater than what Lightning Bolt does, and Lightning Bolt is generally considered the best direct damage spell. Any land destruction effects, bounce effects, or game slowing effects (like Arcane Laboratory or Mana Short) that you use will greatly increase the power of the Vise, and are good by themselves as well. Vise punishes control the most, and control seems to be the plan for the dominant decks in Type I.
The power level of such a card is easily within the "broken" range. Now, if you were to define broken as a card that is a component of a first/second turn kill, it wouldn't be. But such a definition leaves a lot of cards that have had huge influences or even dominated formats out of the broken range...
