I sort of disagree. The UI is clean but things aren't where you expect them to be and it makes it confusing. For instance, I didn't know where to put the subject when I was sending my first email -- it's huge but it's weirdly placed and unintuitive. I also couldn't find the Send button at first because it's on the Nav bar -- not where I expected.
I also agree with the post below re: branding. I went to "Help" to see what they had and it took me to a windows.com page saying "This page doesn't exist" followed by "Hotmail Help". Nice.
+1 on the send button placement. Also when I clicked on the emotiocons, I was expecting a popup to open right below the icon. But the emoticons panel opened up in the left sidebar, totally hiding the to list.
I noticed that about the subject. It doesn't stand out. I get the same feeling from the Mail app for Windows 8. About halfway through this article there is a screenshot of a message being composed. The subject there seems equally hidden.