I agree with the out-of-the box thinkers. When the kids are very young (KG/1st) our GT teacher also looks for excellent readers (although this is not a requirement, it's often an indicator). In the younger grades, they also do a verbal test, rather than a written test.
Also, in addition to being able to withdraw from the program if desired, kids need to maintain a certain grade level to remain in the program. In our school, I believe it's a B in ALL classes (not just an across the board average).
Finally, my daughter has been in the program for 3 years and loves it. It's absolutely her favorite part of the week. They do a lot of fun projects - last month (she's in 4th grade) everyone got to bring in a mechanical item for take-apart day... anything from lawn mowers and blenders, to computers and clocks. With the help of one volunteer dad (for the lawn mower) they took everything apart to inspect how they all work! Very cool! They also learn how to research historical figures, create & market their own inventions, etc. I'm sure every program and grade is different, but they offer lots of fun stuff that really keeps their minds thinking in new ways.