According to this TechCrunch post, Google is going to be improving the user experience of its Android mobile operating system. This is great news, and I'm sure their improvements will go further than simply making the graphics "prettier." In my experience using Android on the HTC Hero and the Google Nexus One, the thing that hurt the user experience more than anything else was Android's poor usability, particularly with respect to navigation. The two big usability flaws were: (1) the use of a dedicated hardware Back button for navigating within and/or between applications and (2) the chameleon-like Menu button that, when pressed, opens a popup menu of actions the user can take within the specific application being used.
First, the Back button, which is the leftmost bottom below the screen on the Nexus One, pictured to the right. My main beef with the way it's used is that it's really easy to get cognitively lost, especially when you start shuffling between applications. My recommendation for the Android user experience (aside from hiring me!) would be to scrap the whole idea of stacking. For in-application navigation, put the "Back" button on the screen, since that's where the user's attention is, anyway. It's how the iPhone does it and it works great. And for between-application navigation (i.e. multitasking), adopt the "deck of cards" metaphor employed by Palm's WebOS. TweetDeck's iPhone client also uses the deck of cards metaphor and it's not only easy-to-grasp, but also sort of fun to use.
Now, the Menu button, which is the second-to-left button below the screen on the Nexus One. From my experience this was even worst than the Back button. The reason the dedicated Menu button is so frustrating is that it's highly unpredictable. I'm actually pretty surprised that Android ever even went with such a confusing feature, as it violates one of Jakob Nielsen's heuristics--Consistency. (Nielsen's heuristics are fundamental to the field of usability.) Since each application will almost certainly have its own set of commonly used actions, access to those actions should not be forced into one single button. The iPhone handles this quite well by letting each application define for itself how to give users access to common actions; usually applications put four or five buttons along the bottom of the screen, each representing common actions. And then maybe the last of those buttons is a "More" option, which brings up a little popup menu.
For both the Back and Menu buttons, from my perspective the key to improving usability is to move functionality away from dedicated off-screen buttons and onto the screen (where the user's attention is). These changes would put Android approximately on par with the iPhone for usability.