As already mentioned, combos are a powerful incentive if correctly used - for example, you can keep up 25 might stacks on the entire group with a minimal amount of players blasting through a fire field. This is like having an extra set of power/malice gear.
Reward-wise, more players means more kills without a decrease in experience or rewards. It's probably more than just a linear progression, since you won't waste time dodging (not to mention you'll never die again).
Also, hearts are completed much faster. To give an example, one of the hearts in Caledon Forest allows you to progress by killing undead. Those undead are spawned by pulling their hands - which are instanced. A solo player can pull a hand, kill the spawn, then move on to the next hand (2 progression points, one for the hand and one for the kill). A party of three can each pull the same hand (instanced), then aoe on the three spawns - not only will they kill faster, each of them will get double progression (1 point for the pull, 1*3 for the kills). While the solo player will have to stay around for a while, the group will pull the 4-5 hands in the area around them and be done with the quest.