It's not going to be lighter with the materials they've used... (at least I wouldn't think so)
Well, the specs are out. It may not be significant, but the Legend is in fact a tad lighter (126g) than the Hero (135g), the Desire (135g), the Nexus One (134g) and the iPhone (134g).
Also the battery life would have to be significantly improved over others in order to justify a reduction in specs. And from the phone lineup we have currently, the less powered phones, don't have nearly enough battery advantages to justify performance disadvantage. So unless that trend changes to a good degree, I don't see superior battery life being a big selling point.
This is what I think: A lower resolution screen (about half the pixel count than Desire and Nexus One, and about the same like the iPhone) requires less processor power for graphic rendering. If this is well balanced, the phone can behave snappier than a phone with a higher-resolution display even though using a lower-performance processor. Less processor power requires less battery.
Similarly, a smaller screen (Legend/Hero: 3.2", Desire/Nexus One: 3.7", iPhone: 3.5"), is, well, smaller and lighter and draws less power. Together, you can have a smaller and lighter battery while supporting the same or even better battery life than a comparable phone with a larger, higher-resolution display.
Moreover, the actual screen resolution in terms of ppi (Hero/Legend: 180ppi, Desire/Nexus One: 252ppi, iPhone: 163ppi) is not so much worse than that of the larger phones (in fact still better than the iPhone): There are fewer pixels, but there is also less screen real estate. Thus, you cannot see as much information at a glance than on a larger phone, but the over all image quality does compare fairly well when you look at both phones from the same distance.
All that said, I am still convinced that the compromize is worthwhile considering, especially when packed in such an attractive and exclusive body. Personally, I find the 3.7" form factor too large and inconvenient for a phone, which (the phone) is still the primary use of these devices for me.