I honestly thought you were joking for a second there, then I realized that you were serious. That saddens me deeply. Let me explain what Halo 2 did for not only the Halo series, but for all modern FPS games.
Pretty much everything you find in modern Halo's began in Halo 2. (Boarding vehicles, destroying vehicles, regenerative health, all sorts of things). Also, Halo 2 was the first Halo to play over X-Box Live. Halo CE's multiplayer was strictly splitscreen, or by connecting consoles. It made use of much larger maps and game modes, was the first game to have Swat and BTB, and pretty much made every Halo game after it.
Halo CE was pretty revolutionary, and it was the first Halo game, but in the metaphorical Halo Skyscraper, it's simply the foundation. The bare essentials. Halo 2 is the beautiful first floor that allowed the rest of the Halo games to come. If Halo 2 had failed, there wouldn't be a Halo 3, Halo 3 ODST, Halo Reach, Halo Wars, or even Halo 4. Halo 2 was the game that put Bungie on the map, not Halo CE. Halo 2 introduced ALL of the brute weaponry, the brutes themselves, the BR, the Beam Rifle, the Fuel Rod Gun, the Energy Sword, and even the Wraith. You couldn't drive any covenant vehicles except for the Ghost and the Banshee in Halo CE. Halo 2 made EVERY weapon available.
As far as innovation goes, Halo 2 was the single most important Halo game that has been made as of yet.
Yes, there were glitches and whatnot, but Halo 2 MADE Halo a series. If Halo 2 had failed, there would not be a series.