When I first played Halo 2 I found the experience boring and to be honest rather mediocre - and this is with the advancements it made over Halo.
From a PC gamer perspective Halo was fairly mediocre fare, and there were many superior PC FPSs already out when it hit the shelves. The thing is on the PC the FPS genre had nearly twenty years to mature by the time Halo arrived, and by that time FPSs were generally more specialized titles. You had multiplayer exclusives like UT and Quake 3, story-driven experiences like the phenomenal No One Lives Forever, straight up run-and-gun like Serious Sam, FPS/RPG hybrids like Deus Ex, as well as other specialized titles like AvP2, Star Trek: Elite Forces, Soldier of Fortune, and Thief II. With the exception of Serious Sam, which was released eight months before Halo, all of these games had been released 1-2 years before Halo.
From this perspective Halo was simplistic and out-dated, and given the many times it had been delayed not surprisingly so.
However despite this it was a hit, and it add insult to injury this bland, uninspiring title was called revolutionary. Revolutionary? Halo? No wonder it was despised by so many PC fans.
Compared to PC fare at the time Halo was very run-of-the-mill. However as a console game Halo was rightfully considered revolutionary as it popularized the FPS genre on the console. While there were some successful console FPSs that preceded it, notably Goldeneye 007, none of them had firmly brought the genre over to consoles.
Halo's simplicity no doubt helped with its success here as many Halo fans hadn't done much with FPSs before this. Sure plenty had probably played Doom and maybe messed around with a Quake title but they hadn't played so many that they were looking for the more focused experiences that PC FPSs titles were aiming for.
While this difference in perspective was probably sufficient to develop some ire between the PC FPS community and Halo fans the situation actually reached deeper than this. Around this time gaming went from a niche activity for geeks and nerds to a mainstream activity enjoyed by lots of people. Many PC gamers blamed the changes in the medium on consoles, and Halo became the poster-boy of this. Consoles may have played a part in this but really computer use in general had gone from niche to mainstream. Many of the people 'new' to gaming also weren't new at all, most had gamed quite a bit growing up but were only now in that treasured teenage demographic targeted by the media industry.
As the industry became mainstream much of that maturing and specialization I mentioned before was thrown out the window. Most people can't just jump in to a game like Deus Ex but they hadn't followed the FPS evolution to get them to that point so developers (consciously or not) started over from the basics. For a fan of more complex and specialized titles this was a travesty.
And again, Halo was the poster child of all of this. Why aren't developers making complex titles like they used to? Because they're catering to the Halo demographic. Why was the stealth system in Thief 3 heavily simplified? Console kiddies. Why is no one making a Deus Ex 3? Because the console gamers don't want a deep and engaging . . . wait a second . . . Deus Ex 3 is in developement.
Complex mechanisms and deeper gameplay are starting to come back, and in force I think. Just as in the 80s and 90s people have been introduced to the industry through more straight forward titles and are now looking for something with more complexity, or something that specializes in a specific area. I don't know if Halo will ever get rid of its stigma in the eyes of PC gamers but I suspect the things they decried about their hobby becoming mainstream will start to lessen in the near future.
