You have to hunker down in cover, throwing frag grenades where the mass is thickest, struggling to hold out instead of confidently advancing across the map. After several missions against small to medium ork and eldar forces, suddenly you're faced with a world-devouring horde. A faction who didn't make the cut in the original Dawn of War, tyranids are locust/dinosaur/xenomorph hybrids who arrive in Biblical swarms.
The difference really comes into its own once the tyranids show up.
