I like the reduction of visual clutter, but I think they've railed too strongly against Android's previous "junk shop" feel, and so there's a new type of mess here: you just don't know what goes with what, and what you can click.
The Widget catalogue list does not look like a list. The icons aren't visually related to their captions. I'd have zoomed in and cropped the icons to a constant size, but I guess the graphics aren't scalable, or the original assets were done on photoshop (tip: if you're designing icons, you MUST use a vector-based tool ESPECIALLY if the size will "never change").
I'm not altogether sure that this is an improvement in UI: the previous Android was thrown-together, but it was familiar: you did know what you could touch. This makes the same questionable decision that Windows Phone has: active areas aren't well delineated, so that text could be text, or it could be a button or a menu, or a tab, but unlike WP7, it doesn't follow its own logic, so styling is no longer a cue: they seem to be using colour to indicate buttons (dialler) as well as selection (tab views).
The fundamental issue with Android hasn't been addressed: there are still different metaphors used in different screens of the UI. Consistency is important in UI, especially if you go down the minmalist route, but that consistency is still not there. For an example of the kind of refresh that Google *should* have done, have a look at Symbian Belle - its UI will be more familiar to someone coming from Android 2.2 than ICS is.
And that wandering "Back" button should be filed as a bug, pure and simple. There is absolutely no excuse for this in a UI that uses a page stack.