DQVI was when they switched developers from Chunsoft to Heartbeat. I don't know how big of a difference that made, as I've heard it had a lot of the same team members but for whatever reason, it just didn't have the same excellence of scenario and pacing as the previous games. The dungeons and puzzles also didn't bring the same novelty I came to expect in the series. Both DQIV and V had a lot more unique ideas in them, although the dream world/real world dynamic in VI was cool. It's a good DQ in its own right but the Heartbeat installments (DQVI and VII) seem to be the black sheep of the franchise - they just don't get the same love that III, IV, V, and VIII seem to get. That might be why they switched developers again, with Level 5 taking over for DQVIII.
Also, one of the features that made it stand out at the time was the graphics, as DQV didn't really have that generational leap you might expect. The team just came off of Chrono Trigger, so they felt they had one last chance to make a title with the excellent art design the SNES library is now revered for. Since we only received the DS remake in the West, which uses the same art assets as the previous games in the trilogy, it more or less gets lost in the shuffle with the more memorable DQIV and V.