Something Halo has, that nothing else has. Tough question, on the obvious side of this coin, their are giant space rings called halos that pose a horrible threat to everything. The enemies are aliens BUT NOTE that there are several different species that have joined together, all going after the humans. That was new; also, space parasite zombies fighting against very strong minded robots. That is just Halo: CE.
Halo 2, you can WIELD AN ENERGY SWORD and PLAY ON THE OTHER SIDE, those are two big things. The plot opens up with many more deadly halos. A new race is introduced early in the game, posing some threat. It also has a CIVIL WAR OF ALIENS FIGHTING EACH OTHER, a MASTERMIND ZOMBIE. The robots rejoin the humans to stop an other halo ring. Also, the brute chieftain Tartarus has a HUGE FREAKING HAMMER. That's Halo 2.
Halo 3, some of the aliens now are on the human's side. Much of this has been done before in the proceeding games, this brings up an issue. The plot is fine: Chief lands on earth, regroups, kills the prophet of truth, finds cortana, blows up the Ark and escapes to some unknown planet. It was a fairly average shooter, from halo standards. New things include: blowing up giant war machines named after bugs, not being able to play as the Arbiter on single player, and blowing up a giant version of a Halo.
Halo 3 ODST, is a different taste of Halo, it should not be played alone as it expects one to have knowledge of the expansive universe. It takes after a mystery adventure about normal soldiers, this hasn't been done in Halo, so it's new and cool to fanboys. It has new characters and an almost irrelevant plot line as the Chief is out winning the war.
Halo Wars, doesn't count.
Halo Reach, not out yet.
Gameplay wise, many people I think are passing over the obvious great parts of Halo:
-The Needler
-Splaser
-Energy Sword
-Forge mode
-Gravity Hammer
-Theater mode form Machinimas,
-Firefight mode
-The sheer amount of vehicles
-The less serious, less realistic gameplay
-Custom games that can become VERY custom
And the general game tactics that can ONLY be used in halo, when was the last time you lowered somebody's with a plasma pistol health to headshot them? How many games have people running in fear of a sword in a gun fight? What other game has vehicles so easily accessible?
What other game has GRIFFBALL?