I will now offer some thoughts on the actual content of the film. If it seems I have dinged this movie, I will offer some praise too.
To start of, I'd say this, people might not realize this, but this movie is rather elegantly constructed. It has a simplicity which is rather appealing. Though you wouldn't know it because of Nolan's befuddling style.
This film is constructed around 5 incidents. Due to the film's palindromic structure, these 5 incidents happen in the reverse order after the midpoint, but they are the same 5 incidents.
1a. Blast in Siberia (mentioned but not shown)
1b. Debicki's encounter with Branagh on yatch (shown in flashbacks)
1c. Opening heist to steal McGuffin
2. Airport sequnce
3. Car chase/road heist of the same McGuffin
3. Car chase/road heist of the same McGuffin (seen backwards - no change)
2. Airport sequnce (seen backwards - no change)
1c. Opening heist to steal McGuffin (not mentioned but you can presume it is happening at the same time.)
1b. Debiciki's encounter with Branagh on yatch (new version - Climax)
1a. Blast in Siberia (Climax)
1a, 1b, 1c are essentially simultaneous events. Everything between these scenes is setup or exposition.
Now look at that, that is a rather elegant construction isn't it? Then why does this movie confuse people so much. More on that latter.
Now on to the time inversion concept of the movie. That is all big ole BS. What this movie is - is basically a time travel movie. The time inversion mentioned, is a side effect of the time travel.
So how does the time travel work? Through turnstile. The concept is this - to go back in time, you need a turnstile twice. So let's say you live from Monday through Sunday. On Sunday, if you want to travel back in time to Tuesday - how'd you do it? You'd go into a turnstile on Sunday and that 'inverts' you. Basically you start travelling backwards in time. So you live through Sunday, Saturday, Friday... etc. And then arrive at Tuesday. Then you would enter a turnstile again, and this time it will point you the right way, so you will arrive at the same Tuesday again, moving forwards, once again. So that's the time travel. It is similar in principle to Primer in that - time travel takes time. Basically in Primer, to go back 6 hours, you need to spend 6 hours in a box. In Tenet, to go back 6 hours, you need to travel 6 hours backwards in time and then start going forward again.
This time travel is different from say Harry Potter 3. Where you turn the time turner, and are instantly transported 3 hours earlier. No. In tenet, it takes time.
So all this inversion stuff confuses people, when this can be explained much more simply.
And finally - what time travel movie is it most similar to. Harry Potter 3 surprisingly. The treatment is exactly the same. Here's how -
Basically, the past has already happened in Azakaban. Meaning, when Harry is dying, a mysterious person saves him. Harry goes back in time to see who the mysterious person is. Turns out it is him! So he was there the first time around too! Meaning even in his first go around, there was a time travelling version existing simultaneously. Meaning he was always meant to time travel.
So in Harry Potter 3 - in the climax - you see a sequence of incidents. And then when your heroes time travel, you see all the incidents again in the same order but from a different perspective and with new information. And there are surreal moments of the regular version and time travelling version interacting. And also subtle changes to result in a more positive outcome for the heroes.
Same is true with Tenet. You see or follow these 5 incidents. And then you follow them again, in the reverse order when the time travel is happening. #3 and #2 you experience the second time when the protagonists are inverted. For #1b and #1c, the protagonists turn forwards again so you experience them essentially normally (though the climax does have both good guys and bad guys travelling backwards in time). Both #1b and #1c also change somethings to bring about a happier outcome for the protagonists.
So there you have it - this is actually an elegantly constructed and rather simple movie. There isn't much more to it than what I wrote above.
No grand mystery. No character development at all (the movie is all plot). If this is such a simple story - why did it take 2.5 hrs and so thoroughly confuse people?
If a person has a great story, and manages to tell it in a way that loses people, the flaw isn't in the story, but in the telling. And herein lies Nolan's fatal shortcoming.
Such a simple accessible story. It could have been rendered in 2 hours. It could have been fun. It could have been easily understandable (and honestly it still is). But you can't deny the sheer bungling it would take to so thoroughly confuse people with such a simple flimsy bare-bones story that people feel they don't follow at all.
I think Nolan has taken to heart that his movies are cerebral puzzles etc. He had a simple crowd-pleasing blockbuster here. He just needlessly convoluted it.
But anyways, I think the film has some merit and some cleverness. But in the end, you have to weigh it against the several missteps it takes to confuse so many viewers.