The relative strengths of various card types ebb and flow - artifacts are stronger in artifact-themed blocks, for example - but it's an open secret that creatures are better now relative to other card types than they were ten or fifteen or twenty years ago. As much as some players complain about "power creep," however, creatures as a whole aren't becoming demonstrably overpowered, but there must always be a generous handful of pushed creatures in every set according to current design principles. But there are still some hard boundaries, says Mark Rosewater on his Tumblr blog.
When thegreatmill asked, "I know you've tried it out, so I'm just going to go ahead and ask. With Memnite (1/1 for 0) printed and Elite Vanguard at uncommon (2/1 for W) Do you feel the 2/2 for G is too powerful for the current Standard environment or are you saving it for something special?" he replied,
Green doesn’t get a 2/2 for G straight up. White I believe can get it with a slight limitation (like say being a legendary creature).
The reference, of course, is to vanilla legend Isamaru, Hound of Konda, widely regarded as one of the better things to come out of Kamigawa block. The green lockout is probably to force a division between the two similar colors, with white as the king of small creatures and green of large ones.