Personally, I thought the first Halo was a little overrated. (Yeah, hardcore Halo fans kill me now.) But I didn't really find the game all that fun. I mean, I thought it was a good game, but Halo:CE just wasn't one of my favorites.
However, different story for Halo 2. I mean, the upgrades just made sense to me and made me fall in love with the game. Dual-wielding, hijacking, and Xbox Live really fleshed out the game.