If you think you may want to try paddling for fun or exercise, there is sure to be a recreational boat just for you, and your whole family. Weight is a factor to consider both in and out of the water. Obviously if a boat weighs less it is going to be easier to tote around and get on and off your roof rack, but it is also important in the water. A lighter boat displaces less water as it moves, making it easier to move through the water. It is going to make a boat more responsive to each input of the paddler.
Stiffness is a factor to consider for longer touring kayaks.
A stiffer boat is a more efficient boat. If you paddle a boat that flexes under you as you paddle, that flexing is using energy that you would rather be using to propel you forward. Durability is a very important factor for most kayakers. Nobody wants to invest a lot of money in something that won't last. Ideally you want a boat that you can accidentally drop off of your roof rack or bash into a submerged rock without worry.