I think this is a great valid question. Here's my two cents on how to make a bike flickable/playful/poppy/unstable (four words that mean basically the same thing in MT

: 1. Shorter chainstays. This has been beaten to death. 2. Higher BB. As others have said, your bike is more stable front to back when your feet are below the axles, and less stable when your feet are above them. 3. Smaller wheels. A mullet is more "flickable" than a 29" bike, and a dual 650 bike is going to be more unstable than a mullet bike. 26" bikes are king...