Funny how only fanboys would excuse the missing features as product evolution. The ambient light sensor and backlit keyboard have been staples of Apple notebooks for the last few generations. Not only is it annoying that they left it out but I'm sure many people bought the product EXPECTING those features to be included. That's where the problem lies.
It's called decontenting, and it happens all the time. Car manufacturers are notorious for it. The ads for the 2011 will point to the new engine but won't mention that they used cheaper vinyl for the seats than the 2010s or removed some feature that they are hoping you don't notice.
I'm most disappointed with the loss of the backlit keyboard, but I agree with those who are saying that Apple is positioning the new Air as the "new MacBook." When Air first came out, it was the most expensive. Now it's available for the same price as the MacBook. The 13" Pro will lose the Superdrive next, and eventually it will be optional, if at all, on the 15" and 17". The MacBook didn't have a backlit keyboard.
Overall, I've concluded that what I gain with the new Air (better heat management, more RAM, better graphics, more available storage) is greater than what I lose (backlit keyboard, ambient light sensor, sleep light, IR).