Bioshock Infinite

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The demos I saw a while back feel VERY scripted, and feel more like an in-engine tech demo rather than the actual gameplay of the actual game itself.

This pretty much. The trailers make it look like a very linear experience. Doesn't necessarily mean it will be bad...but I really hope it will not be a mere "sightseeing tour".

As for the developer quote - that's a very bold statement to make in my opinion. I mean all they've released in terms of Bioshock so far are two games that not only took place in exactly the same world but were also pretty much identical in every other aspect exept for some slight changes to the fighting system.
Saying that there are certain aspects that define a "Bioshock" game, as if that was a genre on its own, when there's actually only one Bioshock game in existence (two identical ones = one game in my book) seems a bit strange to me. But we'll see. Maybe with Infinite they'll kick off a franchise that defines itself by having interesting changes with every installment from now on? You never know...