Although we can speculate about vastly different conditions, the major requirements for a possiblity of intelligent life are:

1. Stability. It takes time for life to form and then evolve to complex forms capable of intellience

2. Liquid water. Basically, the habitable zone is determined by the possibility for having liquid water, since it is an excellent solvent for chemicals and thus plays a central role in the chemistry of living beings.

3. Disaster avoidance. Collision of a planet with a large asteroid, explosion of a supernova very nearby, and a broad variety of other events can wipe life out.

4. A suitable planet - not so small it loses all of its atmosphere, for example.

(cartoon from Gary Larson)