Here's my major beef with this film (and I haven't read the book, so I don't know if this is something left out of it)...
Here you have a botanist up on Mars. Stranded. Alone. Sure to die. And it's never really dealt with how he deals with the isolation. Just right away, he's pulling metal shards out of himself and working on plans and using his poop to grow potatoes. And then (seemingly) 20 minutes later, he's able to chat with the people back at NASA.
Yeah, being in Damon's situation would suck: Everyone thinks you're dead, you can't grow food, you're probably going to die with days, weeks tops. But to me, the worst thing about all of it is that you're alone. And the film never really dealt with how he dealt with the isolation.
This is just nit-picking, because I really liked this film. But a few scenes just watching him, I don't know, pass the time would've went a long way with me.