I like the fact that Treyarch has decided to give the game a theme that focuses on Call of Duty's signature gameplay:
1) camping.
2) running around stabbing with a knife.
I mean, a jungle is good for two things, and I just listed them. Vietnam is full of jungles.
This game is perfect for faction specific killstreaks that reflect how people could play them; the U.S. can focus on air strikes and napalm with in discriminant destruction (no radar please), and the Vietcong faction can have dozens of styles of booby traps from double layered spike pits, poisoned arrow launching tripwires, and classic tree spring noose traps.
The NVA could have tanks and air support as well, but also the ability to create networks of tunnels that all NVA players could see.
If its Vietnam, I'd also like an appropriate soundtrack instead of the tasteless orchestral pieces CoD overuses.
and a new multiplayer mode; Patrol.
The U.S. forces have to make a lap around a map to a pickup point where a helicopter will pick them up, and the Vietcong forces have to ambush them. Kinda like a game of l4d versus, but possibly eliminate respawning for the NVA/Vietcong side to make them more cautious in order to balance it.