Day of Defeat is a WW2 themed multiplayer team and class-based game of capturing and holding off points. Capturing points is not the only gamemode, but most played, if not only one, around. You could get betas of the game for free as mods for Half-Life 1, but the final versions were released as paid Retail product by Valve. And as you can see, it got on Steam too.
First thing first, since it's a multiplayer title... How is community of this old game? It's doing well! It may not have overwhelming amount of players, but you will always find few almost-full servers. And while dod_avalanche map appears to be favourite among the community, you still will get to enjoy some maps variety on various servers. Not so much on game type variety though, capturing the flags points is most played.
As said, Day of Defeat is a WW2 shooter. Team and class based. A mix between arcade and realism. Weapons are as deadly as in many such shooters, just having single person rush forward will do no good, should do that in team. Many weapons also have big recoil, which depends on your stance: moving, standing, crouching or prone. Being prone is the slowest and clunkiest, easy to hit from above, but harder from long distances, also giving character almost no recoil when shooting. Yes, camping is a viable strategy here, but I feel that I have more chance to break through camping than in, say, Counter-strike. Players can respawn any time, so putting yourself to risk isn't bad idea. Each character also have a stamina, which is used when you spring and jump, slowly recovering afterwards. The slowing effect after using it is quite punishing, so you should sprint from cover to cover. Don't use sprint when moving to the front from the spawn point.
Maps are designed to be more of pipes, like classic maps, not open fields, providing a main path and few flanking routes. Even more curious, the maps are assymetrical, which may possibly provide some inbalance (I think that Nazis in Avalanche map have height advantage). Breaks the usual symmetry monotony though, more interesting. Some maps even feature British forces instead of American ones. (they have less classes though)
No, this one doesn't have vehicles, it is not Battlefield after all.
Each team has up to 10 classes. The only difference most have is primary weapon and whether they have or not grenades to spam at corners and choke points. None have ironsight, if you wonder, aside from scope for sniper rifles. And most of primary weapons tend to be quite similiar to each other, which I don't like, as I prefer classes to be more diverce... something what Day Of Defeat Source did. Classes can be divided into:
1) Close Range Assault classes - have guns with light recoil, good for running and gunning, and then getting one-hit killed by another rushing rifleman.
2) Assault Support classes - more recoil, medium range, they better occasionally crouch or stand to be able to litter enemies with bullets at close range.
Close range and Support get mixed a bit together, making other classes.
3) Riflemans - Killing mens with one bullet per few seconds, medium-long range class. Well, and short one too. Only bad when rushed by few mens. Just crouch occasionally
4) Machine Gunners - limited, they can deploy their machine gun on sandbags or while prone. Their machine gun allows them to give your team control over area. Just watch out for snipers and grenade spam. No grenades.
5) Snipers - not really much of different from Rifleman, just got scope and more accurate while scoped. No grenades though.
6) Bazooka - You got rocket launcher. And if you imagine infantry going "whee" from rocket - too bad. It's a utility weapon, which only very few maps have use for, like blowing tank in some map (that I didn't get to play in public servers yet). Very annoying whenever Random keeps giving you this class.
The graphics technically are quite outdated, but still provide it's own quirk. The music is almost non-existing. Only main theme and victory tunes are there.
So, nice game, especially when measured with other HL1 mods. Still fun and still has active multiplayer. But the people who don't like hitscan weapons and who like to get right into action, it's not for them.