All enemy factions have been implied to have rather strict hierarchies. The Fallen are divided into houses and scavenge as clans, implying that you will encounter them in clusters, resulting in short battles if you have strong preemptive strikes. The Hive have loose coordination, probably united by one controlling mind. They will be, as we have observed in videos, fought in waves, wearing you down unless you have good support. The Cabal are heavily armed and have strong coordination, so you will have to wear them down until you can neutralize them tactically. The Vex also have been seen to fight in smaller groups in comparison to Hive and Cabal, though larger than Fallen. They appear to have greater ferocity as well, plus they could be very troublesome if you combat them near one of their gates.