Gonna put all this stuff in spoiler tags....a few secrets I found out about.
Diamond camo? Diamond camo is unlocked through gold camo on all guns in a specific class. If you gold gun all your SMG's for example, then Diamond camo becomes unlocked for your SMG class.
Activision retro arcade in nuketown by shooting heads off mannequins and walking up to the nuketown sign...it will be transformed into a retro activision arcade with about 25 games or so?