As for the ending, I liked it. I didn't see it as a "cliffhanger" that sets up a theoretical sequel, but more as an ending that allows the viewer to simply wonder how events will unfold.
Don't get me wrong. I thought it was a good ending too and I'd rather not see the story continued because, like you said, it allows us to speculate and wonder how subsequent events will unfold. I'd actually hate it if they did a sequel and would refuse to watch it if they did. Some things are better left unsaid (or unfilmed) IMHO.