Good explansion sets not only add new stuff to the base game, but fix problems that existed in it. One of my few gripes with the original Defenders of the Realm was that the game didn't really do enough to throw players on their heels. Granted, there was plenty of randomness to the dice rolls, Darkness Spreads cards, and such, and many times, chance alone made the game far more difficult than it should have been. But for the most part, any plans that players dreamed up could usually be executed at will, with no interference from the game itself. With the Dragon Expansion added in, this is no longer the case.
Saving up a special card for a general battle? Better hope an Imp doesn't steal it from you. Brought along the whole gang to team up on the red dragon general? Too bad for you if Brimstone flies off all the way across the map. Spent several turns traveling to a distant location for a quest? Sure is unfortunate when you draw a Darkness Spreads card that teleports you somewhere else. While these obstacles still rely of randomization, they wreak havok with your plans far worse than the chance elements in the base game. Many might not like it, but I enjoyed the increased tension. It can be fun when things go horribly wrong!
The Dragon Expansion adds many different options for variant rules to Defenders of the Realm. If you find the game too difficult, especially with 3 or fewer players, use the Royal Blessings rules. Tokens, represented boons granted by the king, allow players to stop a general from moving, or reroll all dice in a general fight. If you want a more competitive experience, an extensive fame system variant is included. By doing awesome things, like killing multiple minions at once, for example, heroes earn fame tokens. By accumulating the most of these, you not only get bragging rights, but also advantages in the next game of Defenders of the Realm you play.
I enjoyed playing the base game many times, but the Dragon Expansion really makes Defenders of the Realm a much richer experience. The painted dragon miniatures are cool, an increase in difficulty makes the game truly challenging, and there are plenty of options for variance that add to replayability. The Dragon Expansion is a great addition, and is easily worth a try for those who enjoy Defenders of the Realm.