The army painter function has been part of the Dawn of War series since the beginning. It's a nice nod to the game's tabletop wargame origins. In Dawn of War 3 you can customise each faction with one of a collection of preset schemes that represent the various chapters and orders of the Warhammer 40,000 universe. Or you can set colours individually and create the bright purple metallic Ork army of your dreams.
That's all just cosmetic, however. To change the way your army behaves in battle you can apply three army doctrines, and three elite doctrines. Army doctrines give your ordinary units extra abilities, and can improve their performance in certain situations. Sega has released some examples to demonstrate. Scout Strike will boost their damage when they open fire from stealth cover, for example. Webway Holo-Field causes your webways to project a stealth field—very useful.
In addition each of your three elite units can take one elite doctrine each. They can be quite powerful. The Eldar leader Macha can keep fellow elites alive with the Last Chance doctrine. Gabriel Angelos' Slam Barrier causes dreadnought slams to create bullet-deflecting shields.
Doctrines are designed to be used in combination to create specialised army builds. A Space Marine force that takes Tip of The Spear can reinforce at listening posts—buildings you can construct on top of resource points to defend them. For a relatively slow force like the Marines, the ability to fall back to a listening post is really useful. You could take Pin to give your heavy weapon devastators an attack that locks down enemies, then use Overwatch to use snipers to punish the locked troops with extra attack speed. It creates an extra layer of strategic intrigue in multiplayer.
Dawn of War 3 is out later this month, on April 27. There's an open beta coming, which you can sign up for on the Dawn of War 3 site. Here are some pictures of some funky Space Marines to tide you over, and a shot of the advanced skins you can unlock for your elite units as they level up.