File this under data visualizations and the politics of food.
Without regard to the actual nutritional effects of such a diet (it pretty well mirrors my own when I eat at home, except I go a little heavier on fruit and a little lighter on veggies), let me make a few quick comments about its use as a visual aid.
First, this seems like a good improvement on the pyramid, since it talks about proportions relative to one another rather than in absolute terms like "x number of servings per day."
Second, the US is not the first country to use the plate as a basic guide. (See the gated link to "Comparison of international food guide pictorial representations"
over at The New Republic.) People are pretty familiar with pie charts, so this seems like a good way to go. Pyramid representations are far less common--which means that we have less experience interpreting them and they are probably less intuitive.
A few downsides: 1) it's a little bit hard to tell what the actual proportions are--as one commenter pointed out on Twitter, protein makes up less than 25% of the plate but this could be easily mistaken.
2) Since dairy is a separate circle you don't get a good idea of how much it makes up of the whole. Therefore, even though grains make up 25% of the plate they really make up less than 25% of the overall diet.
3) Does this really tell us much beyond "eat approximately equal amounts of the five basic food groups?"
On the whole though, it has to be better than this rainbow from Canada, which gives almost no indication of either relative or absolute proportions:
If you want to do the patriotic thing with your plate, you could always do as I do on July 4 and eat a red, white, and blue waffle.