Tons of people will agree that Halo3 is nothing but a Halo 2.5 with minor improvements in graphics and gameplay. It has it's frustrating trial-and-error moments in single/multi player. It even has some balance issues with it's weapons and vehicles. Halo3 has a lot of flaws, and that's very agreeable. But there's one thing that makes Halo3 an actual sequel.
Forge

When Halo3 was released, Forge was a useless map editor that was used to move weapons, spawns, flag placements, and all that other stuff. It was a useful tool, but nothing else. That was until Foundry came into play, and it changed Forging forever. Why? With the use of moving immovable objects to create bridges, walls, and entrances, creating your own map was as easy as pie. All that was needed was some patience.

Since then, glitches were found. These glitches gave Forgers the ability to make objects float and attach together. These highly mistakable glitches made Bungie, in my views, a genius. Plus, with the amazing ability to create your own Game modes with the highly customizable options available in Custom Games, Halo3 became the Xbox360's LittleBigPlanet.

Just when things were going great, Bungie wanted to take the term "great" and enchance it to the term "godly". The result?

Sandbox.
Sandbox is Foundry 2.0. It's a large, nearly endless plain of awesomeness where imagination can strive. It's bigger then Foundry, it's better then Foundry, and it's sandier then Foundry. We still don't know much about it, but it's rumored that Forgers have the ability of changing the Day-and-Night cycle of the map. If true, that's wicked awesome.

I can't wait for the Mythic Map Pack. It's #1 on my list for the most awaited DLC on Xboxlive, near GTA4: Lost and the Damned. Lets hope it followed the hype.