Rather than post the usual "because it's popular" post, I've decided to post some thoughts I have had on a popular series.
Halo was a good game. But, notice the emphasis on was. When Halo came out, it was critically acclaimed because it changed things. It raised the standards of an FPS game and the genre as a whole. I think we all have to realize that some games we have played since Halo's release would not have existed without Halo's influence.
But, as time goes on, people expand on the original, and make it better, and raise the standards even more. One could say that Call of Duty 4 was a sort of expansion of Halo's view of the FPS, but it was a better game. The story was better, the game was more challenging, and so on. Use whatever reasoning you want. But now you have the original game, which, although still great for its time, does not compare to the new standards. This creates a bit of a problem. See, now you will have the Halo sequels, which stay on relatively same standards as the original. And some people are fine with that. Those people are fans. They may have varying reasons for being fans - they played the games that came before the original game and were blown away by the innovation, or they grew up with it, or any other reason.
But then you have the people that raise their own personal standards with every innovation that comes along. These are the kinds of people who dislike popular things. They see a series like Halo and all the attention it gets and they ask, "Why does this game still get the attention it does? The concepts are old now. It's been done better. We must move on to the next innovation."
So now you have two clashing groups: the fans and the innovators. The more they clash, the more radical the groups views become. (Such clashes can be caused by high review scores or high sales for the series, or curt words from both sides, or maybe even a stereotype that goes with the fans of the game that the innovators are repulsed by and do not want to be associated with.) The views of the groups become even more radical, and thus these radical views will lead to either devout, Crusader-like fanboy-ism or pure hatred for the series. This will explain the "halo haters" you see. These people do not have something against the series. They just look at a game like, I don't know...let's say for the explanation, Singularity, and wonder why the Halo fans don't give it a chance. "Why didn't Singularity get higher reviews? Why didn't it sell better? It's because the Halo fanboys were too scared to try something new for a change." And this is somewhat true. There are some people I have talked to that the only FPS they have played is Halo. They like it, they are familiar with it, and they don't want to move forward. And this is normal. This is somewhat typical human behavior. But the innovators want to move forward and they get even more angry at Halo fans for their complacence. And then they get angry at the game for having such fans. And views get more radical. And so on.
It's a very depressing chain of events. I myself am an one of the innovators I have mentioned and I have often felt angry at Halo and its fans for not wanting to keep innovating the FPS genre. But I realize that some of the people that play Halo do not see games as a developing form of entertainment the way I do. They do not care as much about video games as I do. That's perfectly fine, not everyone on the earth can have the same views. I just try to remind myself of that fact, and I just respect Halo for its innovation. Sure, it may not be up to today's standards, but tomorrow will bring a new game will raise the standards once again. I just have to respect Halo for its place in video game history. If you still like to play it, that's great. I go back and play plenty of games from the last generation and were innovators of their time. I have my reasons for being a fan and so do you. Do not let the "hate" bother you so much. If you like the game, just have fun, realize why people may not have the same opinions as you, and respect their difference of opinions.