While I agree that the story of the original Halo trilogy was vastly superior to Halo 4’s somewhat pathetic plot. You whacked your thumb with the hammer trying to explain why.
There are a number of reasons Halo 1-3’s plot was deeper, and more epic.
1: Humanity was in a struggle. In Halo 1-3, the Humans get slaughtered when the Covenant are around. They really make you feel like Master Chief is the only thing standing between Humanity and extinction.
2: You are the last of your kind, built for combat, bred for war. In Halo 1-3 you relished in the fact that you were unique. There were no other Spartans left to save humanity, and in that way you were both alone, and a hero. Now there’s an army of slap-stick Spartans at the UNSC’s disposal, and they don’t even take their enemies seriously.
3: The games were subtly epic. And that actually wound up making them MORE epic and less silly. In Halo 4, and pretty much every other fiction 343i has created for the Halo franchise, not only do regular humans never seem to have a problem with the Covenant. But Spartans are grossly more powerful than they were in the 2001-2007 era of Canon, to the point that they are apparently stunt-masters. Doing silly jumpflips, flying off of out of control vehicles “whoah cowboy!”, and seeming simply capable of too much. Every time Master Chief did something particularly epic in the original trilogy, they built a good reason why he had to do it, gave the player the impression that it was a huge risk, and when it worked, it seemed like there was a sense of great accomplishment around the action. Where in new Canon, the Spartans are basically invincible juggernauts and all the epic things they do are a cakewalk.
4: DETAIL DETAIL DETAIL. If there’s anything I’ve noticed about the environments, it’s that 343i got this very pretty graphics engine, and decided that the best use of that engine was flat matte gray/offwhite surfaces literally everywhere. Where is the lovely detail on Forerunner floors, walls, and other objects we saw in Halo CE Anniversary? How do the Covenant ships actually look graphically inferior to Halo 3, a game 5 years younger. And why did Halo somehow transform from a realistic take on the look of 2552, to a futuristic space cartoon? Artistic licence is one thing, franchise integrity is another.