1) Unless you're in management. The pay is very low. Most people never see raises.
2) Most contractors always stay contractors. Even though the're brought in as 90-day to hire. I was there almost 4 years as a contractor. The excuse everyone gets is that it's not in the budget, at this time. From my experience, they never hire people full-time, unless they absolutely have to.
3) No job security, at all.