Well I'm not sure I qualify as old guard, since HBO had an entrenched and active community before I even knew about the site. but i suppose my first post was a decade ago, which is ridiculous. There are two main changes I've observed.

1. the halo community is less active.

2. the halo franchise is less "special".

I think 1 follows 2. Halo was such a cool thing because it was special. There wasn't anything quite like it. It was some kind of social phenomenon that I can't quite pin down. But all of my friends played halo and then halo 2 in high school, and in college ALL of my friends played halo 2. every single one of them. we all got Ds freshman year because our dorm was a nonstop halo 2 lan party. we played on old craigslist CRTs over the ethernet that spanned our entire building. we didn't even need xbox live. and I think this community aspect evolved around halo and halo 2 because they had some universal design language that other games lacked. they didn't have particularly deep metagames - there wasn't too much to learn. and you could just jump in and have fun without being overwhelmed by complexity.

but it seems to me that the games changed and became more complex. halo 3 was sort of the start of this, but it was very gradual. all of a sudden there were unlockables, even if they were only aesthetic. and there were more weapons and more complex maps too, which made the game's learning curve steeper. halo 3 still had 'it' though, and it was still accessible. reach was not. reach is a very complicated game, and so is halo 4. there is just too much to learn to just jump in and play. and perhaps the novelty of the series had finally worn off as other games series' managed to replicate the "virtual couch" formula, and other AA games developed rich lore and complex plots that people could sink their teeth into.

so to summarize, i think halo became more of a niche series, and that's OK. but the community shrunk. perhaps a lot of people outgrew it as well - i kind of did. that's why i barely post here anymore. don't worry, i still read the threads :) I'm OK with all of that. I'm even OK with bungie moving on to destiny (in which i'm not really interested) and 343 making halo fan-fiction games. They're fun enough and that's OK too.