Some memorable new campaigns and scenarios, but that's about it.
Price of Loyalty is split into four campaigns, two campaigns are just as large as the original Heroes II campaigns, while the other two are fairly short four mission campaigns. The first large one, The Price of Loyalty, which the expansion is named after (obviously) is set in the north lands where an Empire has been betrayed by a talented commander. As an old friend of the traitor, and talented in your own right, you are tasked by your superiors to pursue the traitor, and at the same time race against him in order to recover some powerful artifacts.
The second large campaign, Descendants, centers on quite a few generations of a Barbarian family. It begins with a single man struggling to unite the northern Barbarian tribes, and chronicles how that family came into conflict with a rival family thus starting a war.
The first mini campaign, The Voyage Home, chronicles one of the journeys of a knight named Gallant. When his ship is wrecked he is forced to survive on an island, and build up his forces for the journey home. Along the way he runs into pirates, and when he finally reaches home he finds that it has been plunged into a civil war.
The second mini-campaign, and the final campaign included in the expansion, Wizards Isle, focuses around you trying to gain control of a mystical island that has sprung up in the middle of the ocean. You're forced to combat the heroes of your rivals with heroes of your own as you try to find the fabled "Fount of Wizardry".
The campaigns are very well made, and none of them are the same. They also feature very unique missions, so they are definitely worth playing. Besides the new campaigns PoL adds even more standard game scenarios, and like the old ones the new scenarios are also very well made, and quite memorable.
The graphics are the same, but the audio includes some new tracks for each faction's towns. I liked the new town themes over the original ones, but some of the purists did not approve of the new tracks.
The Price of Loyalty is not really anything more than some new campaigns, and scenarios. If you liked the Heroes II campaigns and scenarios, though, The Price of Loyalty is definitely worth tracking down. It may just be easier to track down a copy of Heroes II Gold, instead, however.