Another cool feature: there are notifications in the Google Bar I mentioned before. This could be pretty simple, click it and it says "You've got a new stream post from Bob, click here to read it". But they went a big step further. Clicking the notification icon gives you a dropdown showing your recent notifications (not just the unread ones), *and* if you click one it pops out a mini-view of the whole post (or whatever the notification is about), and you can *comment right there*. In fact you can do a lot of other things in that mini view too, and when you're done you just click the notification button to close it again. Never interrupts your work or takes you away from the other page. Again I really think they need to roll out an optional global "Google Bar" that follows you around the 'net (and no I don't want to have to install the Google Toolbar to get it!).
The Hangout feature I mentioned earlier is definitely a potential "killer feature", but also Huddles could end up being really popular for mobile users. It's basically dead simple group chat on your mobile. There are other apps that do it but this will basically work easily and quickly on every Android phone out there and it could gain a user base very quickly. It's great for planning stuff to do, meeting up, etc.
Deo, you're right about chats and private stream posts and how they can kind of work like direct messages (with offline support). The chat system is exactly the same as the one they've had integrated in Gmail for ages (GChat). That being said they still have GMail, and a full direct messaging system will happen I think, probably just invoking Gmail.
Regarding the "we're over capacity" message, I had that too before I got my invite. When I got the invite it still let me in even though it was saying that on their front page. So... *shrug*
I'm still attempting to use my loophole and it seems to be working *sometimes*.