There are number of aspects of Burning Crusade's design which were significant improvements over Vanilla, one of which was quest design. Quests changed in a couple of ways.
- Quests increased slightly in number while decreasing slightly in grind.
- Quests were split between several hubs in a given zone, rather than tied to a single large hub with a smattering of random quest-givers elsewhere.
While not as obvious as improvements that would come with later expansions, these changes had an extremely noticeable impact at the time. Both dramatically improved the questing experience even if most players weren't analytical enough to explain why questing was better. Less grind is rarely unappreciated, and not having to trudge across the entire length of a zone to find or turn in quests over and over again is a dramatic quality of life improvement.
Burning Crusade questing still had flaws. Quests were designed with the assumption that a player would pick up all the quests at a hub, complete them, and return. Players failing to complete all the quests at once often ended up going back to the same areas of the zone over and over again as quest chains became desynced. Moreover, the area covered by hubs was still extremely elastic to the point where multiple hubs would have quests in the same areas, requiring players to create complicated roadmaps for optimal leveling. This was a mess that Wrath of the Lich King would clean up, but at the time was still a marvelous step up from what came before.
Sometimes you only need slight changes to make vast improvements. It's unfortunate that this lesson wasn't one Blizzard applied in other areas followed Burning Crusade.