Halo 3 builds upon the concepts of Halo 2 in ways that you'd

The Good
Satisfying storyline that simultaneously provides closure while making you wonder what will happen next
Four-player online co-op is a lot of fun
Forge mode map editor lets you do some extremely crazy and unexpected things
Web integration makes sharing new maps and custom modes easy.
The Bad
Rewinding saved films doesn't work quite like you'd expect it to
AI-controlled Marines can't drive to save their lives (or yours)
Inevitable flood of prepubescent online players sure to hamper your enjoyment of the online modes.