What I really like about the side-quests in Dragon Quest 8 is that they don't feel like side-quests at all. This last side-quest felt almost like part of the main plot of the game. To start with, the side-quest is built upon events that occurred within the central plot, and more or less involves tying up some loose ends. While the side-quest itself is a pretty straightforward dungeon dive leading up to a boss fight, the amount of scenes and plot that surround the entire side-quest is comparable to most dungeons in the game.
First off, the side-quest actually introduces two new characters who had so far been only mention in town conversations up to that point in the game. Both of these characters are fully voice-acted and appear in as many scenes as comparable NPCs in the main plot of the game. The dungeon itself has quite a few scenes that take place at the end. Furthermore, the main characters have multiple conversations available about the side-quest, and treat it as their current main goal once the player accepts the quest. All in all, the amount of dialogue, character development, cut-scenes and voice-acting are all on par with any part of the main quest of the game. And this all fairly typical for side-quests in Dragon Quest 8, which makes the whole game feel very consistent.
And at the end of the side-quest, the entire town it starts in changes more than most towns do after a major plot point, with a major overhaul of NPC placement and conversations. So completing the side-quest made me feel like I had really accomplished something, since the game world seemed much for the better afterwards. There is a lot to be said for making the player feel like his actions make a difference.