Stress
You will need to have some well-established stress relieving techniques, or just be the kind of person who doesn't take things personally. You will have lots and lots of patients and very little down time. This is not the job for a clock-watcher; you will be busy.
Since the pay is low, you may have to work two jobs or a lot of hours to make enough money to live on comfortably. Not being able to pay your bills is stressful. Make sure you save enough to join a gym or get a dog.
There's a good chance that some of the patients you have to deal with will become abusive, angry, or violent. You are talking about sticking people with a very sharp needle, after all. It doesn't bring out the best in many.
Finding a vein in those with disorders or dehydration can become stressful. Your reputation is at stake, plus, you really don't want to have to stick the same person more than once. It's not very good for your karma.
Dealing with screaming kids and babies who could run from you, bite, and kick can be stressful. Learn how to handle problems ahead of time.
Also, prepare for the fainters and those who have bad reactions. You're going to have to be cool, calm, and collected no matter what situation you find yourself in. Try meditation during your breaks and off hours.