Warhammer 40,000: Dawn of War is, so far as I know, the first RTS foray into the Warhammer 40,000 universe. Like most RTS games, there are certain things expected of it because of what it is, and while much of the usual RTS baggage comes with it, Dawn of War does a good job of bringing some new paradigms to the genre.
In Dawn of War you are offered the choice of playing one of the four most popular races from the Games Workshop wargame - Space Marines, Chaos Marines, Orks, and Eldar. Space Marines and Chaos Marines, as you would expect, are similar in the basic units you can create, but have different units at the top end of the build tree. Orks and Eldar, on the other hand, are quite different, both in their units, and in the way they play.
The first thing you notice about Dawn of War once you start playing the game is that it's no longer necessary to chop wood, mine gold, and harvest food. Instead, the player needs to claim various strategic points and critical locations around the map by placing a building on them - a building which can be upgraded to add gun turrets for defence. This will then generate requisition points, which can be used to build units. The more of the map, and hence the more strategic points you control, the more requisition points you get.
The other resource in the game is plasma, or energy. This can be gained by building Plasma Generators anywhere on the map, though you are limited to a certain number, or by building Thermal Plasma Ganerators on one of the small number of special locations on the map - and some maps don't have any of these. With requisition points you can build the basic units, to to build vehicles and the like, you'll need plasma.
The nice thing about this is that you don't have to watch dozens of worker units collecting resource for you. You can have just one or two who build your structures, perform any necessary repairs, and then you largely don't have to worry about.
The next thing you'll notice about the game is the unit caps. They vary slightly in the way they work across the four races you can choose from, but each side can really only construct a certain number of infantry and vehicle units. So you can't just be building up huge armies to sweep away your opposition. You have to think more tactically - what units can I build to counter certain other enemy units, where can I best position them, etc. Space Marines units tend to be generalists - they're decent at both hand to hand combat, a big part of the game, and at ranged combat. Orks are all about hand to hand, while the Eldar tend to be more oriented towards ranged combat. So which race you prefer will depend on your style of play.
Whatever you end up doing though, you can't sit back in your base and turtle. In Dawn of War you have to be aggressive, especially when playing multi-player. And it's actually a lot of fun to be out there fighting away while trying to make sure you have your base somewhat defended. That leads me on to the next point though - the fighting. It's AWESOME! Relic went into a lot of detail with the straight out gore of things. It's wonderful to behold. From the gibs that are splattered when units die, to the special death scenes that some of the more powerful units can generate - like when the Space Marine Dreadnaught snaps the back of an enemy unit, and then discards the corpse. It really does bring alive the Warhammer 40,000 universe in such an effective way.
The only things that really let down this game is the somewhat short single player campaign. It's not bad, there just doesn't seem to be anywhere near as much effort put into it as went into the concept of the game itself. Of course, you can always make up for this by playing skirmish games to your heart's desire, but it would have been nice to have something a bit juicier for the single player campaign.
That said, this is a very enjoyable game that has definitely helped to revive and somewhat redefine the RTS genre.