Here's why Halo 3 worked:
Balance, and an amazing campaign.
Bungie took a balanced game, Halo 2, and changed a few things, and balanced them. There was minor bloom, but there was balance. I could predict what would happen next in a battle if I did certain things. Halo 3: ODST also was about balance.
Halo 3's campaign has a very high replayability factor, not including skulls, terminals, and easter eggs. No joke, I can turn on my Xbox, and play through the halo 3 campaign twice in a row, on any difficulty, with or without friends. Halo 4 and Halo Reach has a little of this, but I don't feel the need to return to those campaigns every once in a while. Back in 2007, and the three years of development of it, and even further back, the thing the sold video games was the campaign. Very few people played games online, due to lack of consoles and bad connections. If you make a balanced campaign, typically the weapons were decently balanced for multiplayer as well.
Now, the main focus is multiplayer, and in most multiplayer games now, there is incredible imbalances, why? Because if I know that I can get a powerful gun and mow people down quickly, I will feel good, and want to do it again. But if I am playing a balanced game, I would lose most of my battles, and be mad and not play.
I happen to really enjoy balance. If I know that the reason I died in a game is because of my skill and decisions, and the other player's skill and decision, I can accept that. I can accept being destroyed, although I may not be happy with it. Team work is key in balanced games, and working together, ie a warthog, was an advantage if used right. Now, everyone plays lone wolf, and there is very few times that most team work is thought out. I can go into most objective games in Halo 4 with a friend, and win multiple times in a row, just because we work together.
If I look at where things are with Halo 3. Halo Reach, and Halo 4, I would say 343i has taken two separate paths in matchmaking. There are many elements of Halo 3 in Halo 4, but the imbalance from Reach is dominate. My suggestion and hope for the future: make a wicked campaign, make forge better, and make multiplayer balanced, and if needed, take out things inserted in Halo 4. I want to see a "classics playlists which is balances, and a Infinity playlist, which is more personalized.