As others have pointed out (especially Sevens):
1. Ergonomic grip angle and size -- can be customized by grip thickness, changing trigger length, etc. With a beavertail grip safety and a high hold, the pistol is very controllable under recoil.
2. A trigger that can be made absolutely great.
3. When made right, a very accurate pistol.
4. Very customizable.
5. A bit of mystique.