You start to realize something around the time you've unlocked more armor pieces in Halo:Reach. Mostly it's that you've been playing alot longer than you thought, but it's also that you really haven't had too much of the same experience repeated.
There is alot of variety in Reach, and it's something that will give the game longevity. Alot of these features were in Halo 3 or even ODST, but put together in Reach the game just grows exponentially. IGN put out a strategize where Jessica Chobot told me there was a file she made for me to use for farming achievements. I looked it up, and she was right. Even then I didn't have the same experience she told me I would. It ended up that I modified her modified file, and then I even changed it later, and then told my friends how to change it. The fun result was easy badges for everyone, and I don't think we all had the same game.
That type of thing is really common for Reach. You can expect to be in matchmaking playing slayer one round, then fighting off zombies the next and then getting headshots in HeadHunter (my personal favorite). The matchmaking is vastly improved over Halo 3, and the options, including Forge, make this one of the most robust game offerings in a while.
We're off to a great Holiday gaming season.