The campaigns not like the original at all. You don't have to build a base at the start of every mission. There's no resources. But you still have to play tactically like using cover, flank the enemy etc a bit like Company of Heroes but without the base building.
The 6 squads you use gain levels, this allows you to put points in 4 stats that increase there effectiveness and unlocks new abilities. Weapons drop on the battlefield mainly from killing a boss that you can equip later.
Its way better than Dark Crusade and theres an actually story rather than have to basically play skirmish games.
The multiplayer is almost a completely different game. Requisition points, Power and Victory points are back. So is Population cap. You get 1 base building where all your units are created, theres 3 tiers that you have to unlock before you can make the tougher units. You have global abilities, every time you capture, kill or be killed earns points that you can spend of this.
Each race has a choice of 3 different command units with different abilities, and the global abilities are different depending on which you choose. Each of these commanders have 3 types of gear you can buy for them, Weapon, Armour, Accessory, and each one has at the minimum 3 choices. All units gain levels making them tougher which means you'd be better retreating a unit instead of loosing them, and with the high cost of the Space Marine race you need all the Rescources you can get.
If I remember right 1 single Space Marine uses 5 popcap, 3 for Eldar, 2 for Orks, and 1 for the Basic Tyranid infantry