There are two reasons why I keep games: great gameplay and if they have re-play value. SoM has both. The gameplay is smooth and functional, and combat is easy to master. The primary reasons are because it has an Arkham City combat system that focuses on strikes and counters to gain combos (with the exception of actually killing the enemies). It's the same when it comes to stealth. The ranged system, though, is a lot like Red Dead Redemption with the focus ability. Only difference is that you're doing it with a longbow instead of a gun.
There are two skill trees: ranger and wraith. Ranger allows you to have more capability in combat, while wraith allows you to do otherworldly things (shadow-strike, branding, elven swiftness, etc.) You can also use runes on your three weapons that grant various abilities.
You also have sidequests and other factors that add to the game. You have wildlife to hunt, some of which fight back (warning: do NOT take on graugs until you have absolute confidence in your ability with the bow with and without focus), various plants to harvest for health regeneration, outcast missions to rescue slaves, and challenges for your sword, bow, and dagger.
All in all, SoM is a fantastic game.