There are a couple huge differences between this and Ice Block.

1. Ice Block is a secret. That seems like a small deal, but because of Mad Scientist/Arcanologist, it basically offset the card disadvantage for free. It also enabled Valet pretty much permanently.
2. Ice Block let you "invest" the mana. When you can spend an excess three mana on any turn, and then cash in on it later, it set up plays that this card can't. For example, you can't use this to play Alexstrasza, become immune, then deal 15 damage the following turn.
3. It's in a class that's far less capable of abusing the immunity mechanic than Mage. Some cards are just much stronger because of cross-class synergies.