The Wait & The Hype.
I remember the summer before Halo 3 launched. Summer of 2007. I remember visting Bungie's Homepage and a Wiki dedicated to the Halo Triology every day checking on new info for this game, and every time something new was there, I was amazed and couldn't wait any longer for the game.
The game came, beat the campagin and played it online. I think this is one of the few games that have both multiplayer and single player options that I prefer the single player. Although both were a bit lagging behind, the first time I played both of them I was amazed and felt that the game was definatley worth the wait.
But after awhile, something felt missing, in both modes of the game. I felt that the end to this triology was just lame and poorly put together. Bungie actually reused the final level from Halo: CE in the end of the triology. Couldn't they think of something better. Maybe a massive fight against the Gravemind or something? Although I felt the story was lacking, the gameplay was just Halo, nothing special, a few new weapons, perks and vehicles (The Mongoose was pretty awesome actually.).
However, from the Campagin Mode, I did enjoy the cinematices during the game, voice acting was great and the game looked great. How I expected Halo in next-gen to look.
Multiplayer got old quick, and with CoD 4 launched soon after, many people felt that CoD 4's Multiplayer was far better. (I didn't feel the same until I actually got to play CoD 4 on live.) The customiazble character was fun to mess around with, and just playing the Forge, Halo 3's sandbox mode, with a small group of friends was good fun.
It may have just been me, maybe I overhyped this game way too much, which completely destroyed the game for me, because it wasn't as great as I thought it would have been.