"and no, i’m not a moron."
If you need to clarify it….
"I can name plenty of games that felt very similar to their predecessors"
Then do it. But bear in mind, 2 things: 1-Whether they are part of the same franchise & sub-franchise, and 2-For how long has said franchise been guilty of it and to what extent.
You mention Halo 2 and 3. Let's count the not-minor changes:
1-Forge Mode (the big one).
2-Customizeable armor for both Spartans and Elites in multiplayer.
3-More vehicles, about 6 (technically 7, but one is just an alt. warthog).
4-More weapons and equipment. Specifically, about 11 new weapons and a half-dozen new equipment (like bubble shield).
5-Player 2 now uses Arby instead of a Player 1 Character clone in Campaign mode. Player 1 always uses Chief.
6-In Halo 2, only 1-2 of skulls could be found outside legendary. In Halo 3, all can be found outside legendary (but never in Easy).
7-Some skulls affect your campaign score.
8-Custom Game Types, giving you a great amount of freedom. You can alter movement speed, shields, give everyone active camo, deal higher damage, take less damage, make vehicles indestructable, can't pick weapons up, and many others.
Now, the not-minor differences between Modern Warfare 2 and 3:
1-Can get killstreaks by doing more than just killing (though in most modes, that's the one thing you'll do 90% of the time).
2-About 4 new killstreaks.
3-Killstreaks divided between Assault (classic, powerful), Support (Not reset by death, kinda lame), and Specialist (get perks instead).
4-New game modes: Survival, Team Defender, Kill Confirmed, and on private matches: Infected, Team Juggernaut, Drop Zone, Gun Game, One In The Chamber, Juggernaut, total: 9. Players can customize game modes, but to a severely smaller extent than in Halo 3 (though it does have some creative stuff, like a TPS mode).
5-Hybrid Scopes.
In total, Halo 3 has about 8 additions compared to Halo 2, while MW3 has about 5 compared to MW2. Do note that I did not include some changes for either being expected, or minor. These being:
-Better graphics.
-Different campaign.
-Bug fixes.
-Different soundtrack.
-Different multiplayer maps.
Things added via DLC were ignored for both games.
(Also, if you wonder about the wait between your post and the response: Power went out).