The primary things that people have already noted above are important to test for, including pH, temp., Alk, Ammonia, Nitrite, Nitrate, and even specific gravity/salinity.
Ideally, you will want to bring your QT parameters around to match your main display tank as closely as possible... this will help reduce the amount of stress the fish expereinces when moving from the QT to the display.
You might even want to consider using changewater from your display tank to place in your QT, especially for several days leading up to when you plan on introducing the fish to the display tank... this will help accomplish what I noted above with regard to having the params as close to each other as possible.
-Jimbob