Half-Life is a game full of Valve's genius design, Havok and Gordon Freeman. Halo is a game full of Bungie's sucking up to Microsoft, poor weapon mechanics and Master Chef. I mean Chief.
Half-Life takes you an environment that was well thought out, well designed and overall a immersive environment (less so Half-Life 2, but still, it's better than Halo). There may not be Grunts with exploding confetti filled heads, but it makes up for that with having a guided missile launcher, a crossbow that nails people to walls, likable characters (Halo has one, that one being Johnson), and the scary thing about it in twenty years time I could see those kinds of experiments taking place.
Halo had energy swords, but since when was that new? You didn't even get to make Lightsaber noises with them... the whole Halo idea was also ripped from a book called Ringworld also, and in general just isn't as good as it says its meant to be... come on, having a poor campaign does not give it an excuse for people to say the multiplayer makes up for it and that the multiplayer alone deserves an 10/10 when I myself don't play on XBox Live, and neither, I know for a fact, do a number of my friends.