Short version:
Warning: most of that was totally inaccurate
Halo 1 is a really good game right up until the Flood.
Personally, I liked the change in the combat mechanics and enemy types. It actually brought back some of Doom's spirit after so many years and was a nice challenge in itself with CE's additions to the genre like weapon limitations, the energy shield and grenades - ooooh how much fun it was to watch all those secondary explosions from grenades left on the floor
I like how Cortana was initially not supposed to be hyper-sexualized, and then they said, "Screw it, let's give her enormous breasts and a perfect butt!"
Which is also kinda strange, because they actually talked about how they wanted to keep their relationship with the Chief ambivalent, one that could also be interpreted as a mother-son or brother-sister version.
I guess it's to sell it to the adolescent audience who's not interested in the multiplayer...