So what does this mean? First, Blizzard's system of progressive nerfing is succeeding in its mission. Dedicated raiders are progressing and not hitting a wall like they did in Firelands. (See those initial two months on heroic Ragnaros, where the line is essentially flat? That's a progression wall.) This has served to keep interest up, and I expect we'll see this model of nerfs continued for future raid tiers.
Second, those who argued that Firelands was nerfed too late are likely correct. Only ~2000 guilds had killed heroic Ragnaros when Dragon Soul was released; while that number has kept increasing steadily, it's been rapidly eclipsed by heroic Madness kills. While it's possible that Dragon Soul is significantly more popular, I think it's more likely that many raiders just didn't have enough time to work on Firelands post-nerf and haven't had the opportunity to go back.
For more great analysis breaking down 10-man vs. 25-man and charts of progression for other bosses in Dragon Soul, check out the full article on WowProgress.