I haven't had too much time to explore the SDK directly, and I'm not really a developer anyway, but others have so I'm stealing their analysis, mostly. Three areas of discussion for today.
1) Protective sessions - As described well in the middle of this post from Mobile opportunity, the OS decides which app deserves its memory. If space is short, or it stops trusting what an app is asking for, it could be shut down without warning. Loss of user data is annoying (especially on mobiles, where it might have taken a bit to get it in there). My S60e3 phone regularly shuts down apps in the background (why?) and its annoying as hell, especially when I am halfway through a form.
2) UI stuff - Engaget has a slideshow from the SDK simulator if you haven't installed it yourself. Some issues and inconsistencies. Too much pretty stuff, too much reliance on side-scrolls and tabs, but its very early so we'll see where it goes. This one exemplifies all my fears of low- and no-affordance UI. The menuish hard-labeled key is sort of a single "options" softkey, but rather tenuously related to the menu that flies up. And very pretty layers appear when you sidescroll to submenus, but they are also only tenuously related to the previous menu. But its still very early.
When I think of open source OS, I think of the range of GUIs (and even ascii shells) for desktop Linux. Not expecting that from Android, but how much individualized GUI customization will we see? Assuming plenty, the good is that some device makers or operators will really excel with their own changes. The bad, of course, is that the GUI is the OS to the everyday user; changes mean itslonger the same for every user, or even if you switch phones and try to stay in the same OS.
3) Marketing - One aspect that hasn't been touched on as much is how to sell this to consumers. I have a fair number of techno friends. The best so far was the friend who was sad that Google hadn't released an actual android. Overall, when I start talking about this, no one I know outside the industry actually cares.
It could be that everyone I know is too young to have a phone, or as old as me, but no one actually downloads applications, or customizes their phone past ringtones and screensavers. Presuming a future where every phone has a neat, computer-like OS, they will not all be the same. Will all the marketing to get share be directed at the operators?