One of the most important things is a [full tang][1]. The tang is an extension of the metal of the blade into the handle. In knives with a full tang it goes all the way through the handle. This improves stability, control, and durability. Cheaper knives with partial tangs will have the handle break off over time.

In a chef's knife you want a blade from 8 to 10 inches long, whichever is more comfortable.

This brings me to one of the most important points: _It must feel good in your hand_. Don't buy an expensive knife just because it's expensive, or you recognize the name. Go to a store that will let you use the knife, Williams-Sonoma is one that will do this. If you can, bring a carrot in your pocket and actually cut that in the store.

<sub>I'll do some research on steel quality and update this later.</sub>

[1]: http://culinaryarts.about.com/od/knivescutlery/ss/anat-knife_5.htm