1. It's going to take a long time to get there and the journey might not be very fun.
2. Like, really not fun.
3. Humans have never been in space for that long before.
4. It'll be hard to adjust to Mars time.
5. You'll barely be able to see the Earth anymore, which might affect you more than you anticipate.
6. Once your body has adapted to Martian gravity you won't be able to come back to Earth. Ever.
7. The first Martian settlers won't be able to have children.
8. You'll have to exercise to stay in a good condition. A lot.
9. Especially as, if you get ill, you'll be (on average) 225 million miles away from Earth.
10. There's always the chance that you'll get infected with something from Mars.
11. You'll never go outside again.
12. It will take between 3 and 22 minutes to relay information to Earth from Mars, so a phone call to friends back home is out of the question.
13. Which also means you won't really be able to use the internet.
14. You'll never get to eat your favourite foods again.
Unless they're made entirely of certain vegetables like sweet potatoes, spinach, lettuce or soya beans, which are plants we know can be grown in space, so should do fine in the reduced gravity of Mars.