I feel like the only criticism I read is "it's not Portal". A puzzle game having an open world isn't bad at all. Characters not being portal characters isn't bad either. You don't explain why the themes of the story don't fit the game.
When a puzzles games has excellent, great puzzles and is long, it deserves more than a 6 because it's not Portal.
Not necessarily. "Not being Portal" is honestly a pretty big problem for a lot of puzzle games, because, well, most puzzle games are quite boring.
Portal had interesting puzzles, but it was really the humor and story and general feel that made it what it was; the Talos Principle lacks humor, the demo didn't impress me with the story, it had a good feel to it, and the puzzles in the demo were meh - not totally awful but nothing I hadn't seen before in various forms.
So... good feel, meh on everything else... yeah, not being Portal is not unfair.
I mean, let's face it - this thing's metacritic score is horribly inflated. This is probably the only honest review of it. Sadly, it doesn't really go into the flaws very well.