Frugalchief has made some excellent points, thought I'd add a little from my own experience working in real estate.
-Beware the temptation of leverage. A lot of real estate deals look great when you are purchasing the property with leverage, and since it's so easy to get and use why not. Easy access to financing is like picking up girls at the bar after a few too many, they all look good. Does the deal make sense if you're not using leverage, if yes then it's probably a pretty good deal.
-Real estate transactions are like a fresh kill on the African Savanna. If you're the lion and you went out and found the deal, rest assured there's a mixed lot of industry professionals scavenging for their share too. You can go out and buy a couple million dollars worth of stock for a couple bucks through just about any of the major brokerages, if you spend that much in real estate you're going to be paying in the tens of thousands to the realtors, lawyers, home inspectors and other professionals involved. Assemble your "team" first, let them know you want to invest in real estate and that you'll stick with them and provide more business if they keep their costs reasonable. Have your team lined up before you commit to a purchase, they should be able to help you find a good deal.
-Be willing to get your hands dirty. If you're paying out of pocket for repairs and to have someone else do them that quickly eats into your profits, especially if you only have a couple units. As you scale up its easier and more necessary to have someone else do the little stuff.
Take a value investing mind set to real estate and try to keep the costs under control and you'll be fine. Many people jump into real estate because its easy to do and everyone else is doing it without taking a hard and pessimistic look at the deal they're getting. If you still think it's a good deal when you project in vacancy and below market rents and thieves stealing everything from the place, including the kitchen sink >:( then you might be on to something. Buy with a margin of safety and build a margin of safety by finding good tenants that add value to your investment.