Something with my experience with the N900 is that if you want this OS to be functional as the phone, the phone app needs to be able to respond to human input quickly. At times, because something in the background was eating processing cycles, I haven't been able to answer a call because the phone app wasn't coming up and allowing me to respond.

Now, we can talk about how everyone should be well behaved, but the customer doesn't care about that. They care if the device is responsive.