Who will win?
In M12, Giant Growth will be a Giant Disappointment, mostly because it no longer exists. Giant Spider will be the only card after this summer that's existed in every Standard environment. It's the only card that's been printed in every core set from Alpha to M12. It raises a very good question.
Originally, Giant Growth was green's answer to Lightning Bolt. If the growth leaves the format, Bolt's future is shaky at best. The lack of Giant Growth will greatly change the shape of limited, for sure.
It also says something for Infect. Will the mechanic have any future through Innustrad? Infect builds have been picking up in popularity as of late, and the G pump spells are a solid staple to make InkmothNexus.dek work. The deck, using four inkmoths and four Expedition Maps, gets a Nexus in play and attacking bu the third turn, then pumps it with all excess mana for a hopeful win by T4 or 5. Without Giant Growth, this becomes significantly harder. There's still Groundswell, Unnatural Predation, and Vines of the Vastwood for now, but when Innustrad hits all of that goes away (in addition to the map). If this deck has a future, it's a very shortlived one.
That brings me to my last point, the hole in the slot. Giant Growth is flavorful, iconic, and simply put, an amazing design. If Giant Growth is removed from M12, another card has to take it's place, and the card will be very similar, possibly even a functional reprint. Perhaps Unnatural Predation will be in M12. Perhaps something else. It's all speculation at this point.
My design sense thinks that there could possibly be a functional reprint. Most of Green's creatures are already big, making Giant Growth a bit redundant. You can only make a creature so large before size becomes a weakness. It's entirely possible that the new Growth is a functional reprint, but something that makes more sense when stacked together. Wizards R&D have been focusing more on the Vorthos part of magic since the player base has been maturing, and they've shown that they're ready to make these decisions to impact the game's flavor, like with Remove Soul turning into Essence Scatter for M11. They do the same thing, and Remove Soul was an iconic part of the game, but it still had to go due to minor flavor reasons- how could you take out the soul of something that wasn't even on this plane yet, and do it for only two mana, AND have it be in blue? Removing a creature's soul is a black effect, not a blue one, and the name of the card simply didn't fit its game mechanics. So despite it being an icon over nearly two decades of Magic, it had to go. The same could be said for Giant Growth.