I think a lot of the hate comes from the fact that Halo was THE first person shooter to beat, but then CoD took off with Modern Warfare and it's more visceral (and by comparison) more realistic gun fights and it's multiplayer progression loop. It was also multi-platform therefore started selling a ton. Suddenly, Halo was no longer the best selling FPS. While I definitely prefer the more skill-based play in Halo, I have a lot of fun with CoD. Ultimately, I always end up wanting to go back to Halo, but some of my friends only play CoD, even though they were beasts at Halo.