To make things clear, this is not a topic why I think Halo 4 or any future games are not good, but why I think these games will never been considered good by a majority of the community.
When Halo 3 came out, online play was a new experience for a majority of gamers. Yes, Halo 2 had online but it wasn't really mainstream until this gen of consoles came out.
So do you it's a coincidence that most most people think Halo 2/3 are the best in Halo series and CoD4 is the best in the CoD series?
Now I attribute this to two things: Firstly, the unreal experience everyone had playing massive games with their friends and other people for the first time is gonna stick as their "favourite game of the series." Now of course, this doesn't apply for everyone, and it is not the sole reason. Of course some games are going to have better gameplay mechanics then the others, but I think the first game played in a big series will always hold a biased spot for most gamers.
The second reason is a much bigger factor. The mainstream FPS genre is just getting old. And when I say old, I don't necessarily mean boring and no one wants to play it anymore. (like the whole guitar hero phase)
I mean that over the past five years, internet activity for discussing, reviewing, criticising, comparing etc etc games has exploded. Diverging opinions are EVERYWHERE. When the first online FPS' came out, all the developers had to do was make the game. Now, they are faced to not only live up to their past titles, but to please masses of different (whiny) needs of the community.
This leads a majority of people to blindly follow the lead of others who say "ahh Halo 4 sucks because of this, Halo 4 sucks because of that..." The one thing I hate about the internet so much is it makes it so hard to have your own opinion about a lot of things...
tl;dr, the series has too many strong diverging opinions and too many people that will believe anything they read on the internet.
In my case my first online game was Halo 3 which I loved and spent a lot of time in it and that's where I met most of my xbox live friends. Then came CoD4 and I just fell in love, that's my favorite game of all time (thanks to Search and Destroy).
Now no one can't deny that MW2 was garbage with Commando, OMA/DC, OP shotguns as secondaries, easy to get the nuke etc. Black Ops 1 was OK, had it's flaws but most of them were patched. MW3 was terrible thanks to lag compensation, it was fixed but until January. DMH was BS, and all the Death Streaks, although it had good things like specialist. Spawns were always awful BO2 is garbage, again thanks to lag compensation, bad spawns, a million places to head glitch, uav every 30 seconds etc.
Now Halo Reach... won't get into it, I'll just say I didn't like it, didn't even play for 2 days (game time). Now Halo 4, I love it, I think Halo evolved and there is no way I would go back to H3 after playing H4, unlike CoD, I would love to go back to CoD 4 but it is full of hackers and not a lot of people paying it.
i base my opinions on good experiences in the game. halo 4 for me is a good experience in the game, so i prefer it, the only other one i've played online is Reach, and i didn't like that too much, i have anniversary, 2 and 3 and ODST but never played online with them, so reach and 4 are my only comparisons, and 4 is better by a long shot, but as you said, your first online experience will always be a favorite, so in that vein (shoot me) COD MW2 will always be the benchmark that i compare any online game to, luckily MW2 was a pretty low benchmark so theres not many games i think suck online.