you're right the Source engine is dated.
but I still fire up DoD from time to time only because it's one of the most active online communities for a WW2 game. most WW2 shooter multiplayer communities have dried up as the franchise moves on, but DoD servers still regularly fill up with real people; no bot support.
the only other recent WW2 game with any kind of decent online community I can think of is Battlefield 1943 but it's also multiplayer only. no SP campaign.
Wolfenstein 2009 is a good game but it's not traditional WW2 of course - it's got zombies and mad scientists and sh*t.
if you're familiar with the Commandos franchise, Strike Force is also a good WW2 game although slightly dated as well.
Hour of Victory is ok but pretty short.
The Saboteur is an incredible game but it's not a first-person shooter and it's not really based on WW2 battles.
it's about an Irish spy working against Hitler during Nazi occupied France. it's basically Grand Theft Auto meets Assassin's Creed (climbing buildings, rooftops) but if GTA was in 1941 and all the cars had a top speed of just 60 MPH.
Velvet Assassin is similar to The Saboteur except you play as a woman.
you might also keep an eye out for Heroes and Generals or Enemy Front coming in 2014.
sorry I really don't know much about Viet Nam games.
they never interested me as much as WW2.