Stress
Any career that involves such a great deal of risk, especially monetary risk, especially monetary risk involving other peoples' money, is sure to come with some pressure. And until you've reached a certain level in your career, you'll feel that pressure on a fairly consistent basis.
Not only will you have to stay in the black at all times, you'll have to continuously do research to figure out where your next business move should take place.
Oh, and keep in mind that things are bound to go wrong every step of the way. Your building permits could be late, your lawyer could miss signing a crucial paper, someone else could put in a better offer on a house you wanted to flip, etc.
Even aside from the sheer amount of money and the daily imperfections, you'll be stressed out by all those involved, from your bosses to the city council to the investors. People will be breathing down your neck to get answers, estimates, numbers, and reports. It's enough to make your head spin.
So before you decide to become the next big real estate tycoon, make sure you can look down the business end of a metric ton of stress without breaking a sweat.