1) There isn't much of a plot which was fine with me. I could look at beautiful animals and amazing aerial photography all day long. If you need a movie with a plot, Earth probably won't be for you.
2) I would advise against taking very small children to see it. While it is rated G, there are a few slightly graphic scenes depicting the demise of an animal. Young children will most likely be very frightened by these parts.
3) I think Disneynature could have used this opportunity to have a bigger/better call to action to promote conservation. The narrator (the lovely James Earl Jones) talks about this a few times, but I think it should have been done more throughout the movie. There is a fine line though because you don't want to go overboard with this and ruin the movie by using it is a soapbox. I think Disneynature just decided to play it safe with Earth.
When you see Earth, they show a preview for Oceans which will premiere on April 22 (Earth Day), 2010. Great idea! Oceans is similar to Earth, but it is set underwater. I already have it on my calendar :) I hope that Disneynature uses Oceans to really stress the importance of conservation. What a powerful platform!
I give Earth 4 out of 5 stars. It is a beautiful and engaging film about the different species of our wonderful planet. Go see it today or tomorrow, and Disneynature will plant a tree for every ticket sold. Together, we can and will make a difference. Let's save our planet from our own destruction. Borrowing the words of Gandhi, "Be the change you wish to see in the world."