There are several qualities by which keyboards are judged. Here's a quick overview:
The most important thing to look for in a keyboard is a solid key-click. When you press down on a key, there should be very obvious feedback when the key is depressed. Good keyboards provide feedback by the means of a clicking sound, a solid tactile click, or both. Cheaper keyboards don't feel as responsive, especially cheap membrane keyboards which have a mushy, sponge-like feel when clicked. Commonly referred to as tactile mechanical feel keyboards, these tend to be somewhat loud, especially in office or dorm environments. Examples of these types of keyboards are the older-style IBM Lexmark keyboards (also used on IBM typewriters), the old Northgate Omnikey, the CVT Avant Stellar (new version of the Omnikey Ultra), and the Focus 2000-series keyboards. Another group of keyboards features positive feedback but is much quieter and also tends to be softer to the touch. (i.e. they require less force to press a key) Popular keyboards of this type are the Apple Extended, Microsoft Natural, Sun Type 5, and the Keytronic Lifetime Series keyboards. The choice between the two styles is a personal one, and is sometimes dictated by the environment where the keyboards will be used.
For gaming, one of the most important qualities of a keyboard is being able to hit multiple simultaneous keys. For instance, in FPS games, you want to be able to hold down the forward, jump, strafe, and run/walk keys at the same time. Cheaper keyboards only allow 2-3 keys to be pressed at once (anything else is ignored, which means that while you're running forward, you may not be able to jump at the same time. Pretty noticeable after a few minutes). This really becomes apparent in some multiplayer games where two players share the same keyboard (i.e. Street Fighter or Mortal Kombat). Most good keyboards allow for 4-6 keypresses, and some don't have a limit at all.