I understand why they had Lewis do the final EVA, but it really undermines her character for me. She's a tough-as-nails former-military commander, there's no way in hell she'd pull the EVA specialist off the most precise EVA they have to make. Lewis essentially puts everyone else at risk because of her own guilt, and it's a shitty way to end an otherwise great character.
I get why they did it, but the overall changes to the ending bugged. More so, I think, because in the book Watney explicitly says "Now, if this were Hollywood, here's what would happen...but this isn't Hollywood, so now here's what actually happened." It seems like they threw out a large chunk of why people loved the book so much. And I love the film, so I don't want it to sound like I thought it was terrible, I just wished it had skewed a bit closer to the source material on some things.