After watching Tenet, I am quite puzzled. It's not a bad movie. It's well directed, has its peak moments, operatic and virtuosic. The actors do a really good job.
Then where does my feeling come from, this feeling that the movie will vanish from my memory like the scent of good coffee while you move away from the roaster?
First of all, the plot. It is interesting, even a bit fascinating. A little bit less, though, if you are used to reading scifi novels. Plus, in a novel you can always reread the complex passages. Here, the start is teasing but it becomes so increasingly tangled that you are lost.
And the worst part is: you are lost and you don't care, not anymore than you care about the fate of the characters, because (and this is the second reason) Nolan, if he masters action and can direct a war scene like a ballet director, can't depict real characters. Do we believe in the budding love story? Do we care about the friendship between the two male characters? It could, should be moving but is awkward. We don't even see the kid who is at the centre of an emotional balckmail! Nolan is too focused on his obsession for time to take the time to render his characters interesting. In all his films, he lacks empathy, just like Martin Scorcese (a giant!) can't shoot a sex scene.
The result is, after some time, we are lost in the labyrynth and don't know or care who dies. Just like in Inception, we feel we are in a video game, a giant Call of Duty, where the hero always has as many lives as he wants. The problem being that when or if the hero dies, who cares? It's just a game.
If Nolan, like in many movies, could go back in time, he would learn that characters are the key, not CGI! And would maybe avoid making the same mistake again.

My third reason is: it is way too talkative and thus loses a part of the possible secrecy, mystery and even poetry that it could convey. Watch what Tarkovski, in Stalker can do on the topic of distorted time, future, past, destiny, irridiated zones! Watch and listen! Silence can be so effective!

My fourth: It is way too long! Again, there are good ideas. Connecting the "reconcialition" of Kat and Andrei (fantastic Kenneth Brannagh!), their possible hug and kiss to the explosion of a bomb is seducing. But again too long full of useless dialogues and not surprising because we expect the end. In Touch of Evil, Orson Welles did the same in 3,12 minutes, with three words of dialogue and it was mindblowing, a masterpiece of a scene!

Finally, Nolan misses talking about cinema. Indeed, in the movie the characters live the same events differently (Edge of Tomorrow already developped the idea). They change angles, point of view, perspectives, confront themselves. Well Nolan could have realised that this is precisely what cinema and actors do, all the time!

I still watch Nolan movies but always ends up being disappointed, as if Nolan was not following a scenario but fulfilling a programme.