Stability is the first thing most people notice in a kayak. This has a lot to do with personal comfort level and plays a major role in boat selection. But aside from any stability characteristics of the boat, what is stable for one kayaker is not necessarily stable for another. A person's physical characteristics, experience and intended use create a lot of stability elements in their own right.
Starting with the boat, we can differentiate between initial stability, secondary stability, and final stability.
Initial stability is the amount of rolling resistance the boat offers in the first few degrees of heel. It is the amount of tippiness when you first get in -- more of a comfort level rather than anything else. For novices, initial stability often dominates their idea of stability.
Secondary stability is the point where the boat stops becoming more stable with increasing heel. Any further tilting would be more de-stabilizing. This deals with the boat's ability to recover from sudden leaning, fighting large fish or other upsetting movements. It is also significant in rougher waters where more lateral control of the boat is called for. In this case, too much is not necessarily a good thing.
Final stability is where the boat no longer has any resistance to capsizing.
Boat length, width, seat height and hull shape all have a great influence on elements of stability. They interact to such a degree that it would be misleading to make any blanket statements here, but can easily be fodder for another topic.
Effects of the kayaker: Heavier and taller kayakers tend to influence stability negatively by raising the center of gravity. So normally a small person would find any given boat more stable than a large person. A person's sense of balance and previous kayaking experience are also important in any consideration of stability.
Kayakers have some tools to improve stability with their boats. The paddle is used to help balance and brace with traditional kayaks, so transitions to a Mirage Drive Hobie can be disconcerting at first, even for experienced paddlers. On the other hand a relaxed, flexible upper body and the ability to lean or edge can make a major positive contribution, especially in rougher water. Handles to grab can keep you from sliding or getting tossed when faced with a beam swell. Let me also mention seat height. Some users employ a second seat pad on top of the standard seat, thus raising the center of gravity. On narrower boats like the Adventure, the difference can easily be felt with the boat's tendency to heel when sailing. As one who likes to ride boat wakes, I use the i-comfort seat in place of the pad and then let some air out to lower my position -- significant help when trying to keep the boat upright along the side of a swell.
The bottom line is, if you don't feel stable, you won't want to use your boat. When demonstrating a kayak, take the time to sit in it, move around, hang your legs over the side, and get the feel for the boat's limits. Get past the initial stability stage in shallow water, wiggle and rock the boat out to it's limits. Going for too much stability can be as big of mistake as too little, as you tend to lose the ability to lean your boat when necessary. Again, someone else's take on stability does not necessarily apply to your situation. Above all, you want to be comfortable with the boat you select and safe in the waters you cruise in.
A more comprehensive discussion, including design elements can be found in this article:
http://www.guillemot-kayaks.com/guillem ... _stability 