I'm trying to get better at critique and Interstellar is one of the things I decided to write one for because it's still in fairly recent memory. So whatever!
The main problem I have with Interstellar is that everything that happens in it feels really contrived. It feels like a film written in sequences which simply had to be in it, with the connecting pieces added later. This often means that the film tries to feel smart in expense of the characters being stupid.
Now, I don't mean that characters lacking in valuable insight or intelligence is necessarily a character flaw, but in this film in particular it seems that characters act simply according to what the audience should see next. For instance, Cooper had to be shot into space on a mission without formal training or even basic briefing, despite the inevitability of the mission taking over a decade at minimum to complete. It's not because the people at NASA didn't deem any of that to be necessary for such an important mission, it's just so the audience would be treated with the hammy emotionally loaded leaving scene and establish the bitter relationship Murphy has towards her father for most of the rest of the story. It also leaves open the opportunity to have all the technobabble that makes this film ”scientific” be explained in plain exposition later when the mission is actually briefed. In space!
Then the crew gets to Miller's planet, and something fascinating happens. There's been a seemingly promising consistent transmission coming from it, so the crew decides to check it out. And then it's explained that time is extremely dilated on the planet, so it's going to take years just to go there briefly. So, when the whole thing expectedly ends in disaster, Cooper asks why the wreckage of Miller's ship was still on the surface despite the huge waves, and Amelia doesn't hesitate to answer the obvious: Miller would have only just landed there herself in her frame of reference due to the time dilation. Yet this simple notion is completely ignored by having the crew as well as everyone in NASA think the transmission is indicative of any eligibility of the planet, despite every character knowing fully well how time dilation works, except when it actually matters. They had all the knowledge and tools at their disposal to find out why the transmitter isn't worth retrieving, and they could have easily found out about the planet's enormous tidal waves from orbit if they had just taken a moment to study it. But this doesn't happen, because we need to show the audience a cool setpiece and age the characters on Earth to where we want them to be in the plot. Again, it's not so much that the characters are acting stupidly that is the issue, but that it's contrived for the sake of plot convenience and neat-looking visual backdrops.
The technology in this film feels really contrived as well. You can see there was a deliberate effort to make the film grounded in reality when the crew leaves the Earth. It's realistic and easy for the audience to accept that space technology wouldn't be hugely improved from the present day, so it's immediately established that it hasn't gotten Star Trek levels of convenient – at least not at this part of the story. They take off in a multi-stage rocket and take two years to get to Saturn and it establishes a notion that space travel is still really difficult and interstellar travel by normal means is still virtually impossible.
But then they go through the wormhole and everything changes. Nolan seems to have decided that different rules apply to different places, because as soon as we leave the frame of reference of our solar system, the film takes a dramatic turn from science fiction to space fantasy. Travelling between planets seems immediately much more effortless, and taking off from a planet on a shuttle is something they can actually do all along. So much for grounded in reality. Maybe they had a legitimate reason for taking so long to get through the wormhole and use such an antiquated process for it, but to me it just feels really contrived that they create an expectation for one thing and then proceed to immediately drop it when plot convenience deems it necessary.
Eventually the tesseract happens, which is a nice visually stimulating image to serve as a backdrop to Cooper's exhausting delivery of exposition that conveniently explains everything that is happening in the scene. And then Murphy makes the bizarre assumption for no reason at all that her father is the ghost in the bookshelf and happens to be completely right about it. Everything wraps up perfectly, and that's just great. But also really, really contrived.
I respect Nolan as a director, because he goes to enormous lengths to achieve his vision. The amount of practical effects used in this film where CGI would have sufficed is insane, and it's great that he's willing to go through the effort. It's just a shame that his vision seems to only be limited to visual imagery, and how we get to see his visual setpieces isn't as important to him as simply having us see them. Perhaps he would benefit from having a screenwriter to closely work with and iron out all the weird contrivances and trim the heavy-handed exposition that stop his pretty good films from being great films.