The answer to this question is definitely subjective. Anecdotally, my dad, my brother, and myself have all read the book and seen the movie. We've asked this exact question to each other, and all three of us provided different answers.

One thing that we all agreed on was that we wish the movie had done a much better job of conveying Mark Watney's sense of humor. It is a very important part of his survival, and the book even spends some narrative time explaining that it was Watney's sense of humor and ability to laugh and move on that got him selected for the mission in the first place. Damon did a wonderful job with the character, but he just couldn't convey Watney's personality in the same way that Weir wrote the character. There is a reason for this. Much of the book is written from the first-person perspective. This is one major thing that I think the movie is simply missing, but that isn't the fault of the film. Conveying the first-person perspective to a movie audience is nearly impossible. The consequence is that audiences do not get to see and understand the thought processes that go through Watney's head. We see him do stuff, but we never fully understand the how and the why. The majority of the real-world science is missing from the film.

The film does include a short ending sequence showing Watney's return to Earth, and this is one thing that I do wish that Weir had written in rather than ending the story with Watney rejoining his crew and heading to Earth.