Actually the difference is not that much, if we're talking about hybrids with with front suspension, which are rather close to MTB's. A hardtail MTB will have lower gearing, more compact frame, more suspension travel at the front, and will have/be able to accommodate wider tires. Probably also, especially from mid-range and up no fender and rack mounts. Many of hybrids use the same drivetrain components and brakes as MTB's.