Tim Morgan has similar thoughts to me about the Mac App Store, adding that the back/forward buttons don’t stand out (unlike in Safari), meaning many users will consider them greyed out, and the login sheet has irksome wording and poorly considered design elements:

Firstly, “Billing Info” is a noun and button names should generally be verbs or verbal phrases. (What am I doing with my billing info when I click this?) Worse in my mind, however, is the “Forgot?” hyperlink. I’ve been seeing hyperlinks (or faux-hyperlinks) pop up in applications here and there and I’ve never been a huge fan of it in desktop applications. Much like on the Web, these hyperlinks come in different colors, sizes, and styles; here in the App Store it’s just blue text (not even underlined!) that could easily be mistaken for a plain static text element (especially by the colorblind).

Morgan also dismantles the new Twitter 2.0 app’s UI, exploring its many odd decisions and trying to decide if it’s good that devs are experimenting or whether they should conform to standards. John Gruber also offers an excellent take.