Looking at previous expansions, I was struck by how little the cycle has changed between Cataclysm and the current expansion. The only real difference is that Cataclysm had one patch (patch 4.1) that was a smaller patch with dungeons and world changes, whereas Mists of Pandaria has had two, patch 5.1 and 5.3. (I say smaller, but that's not really an accurate term - it's simply more convenient than typing 'focused on non raid content' over and over again.) So when we talk about the rapid patch cycle of Mists, we're basically talking about one patch.
So, therefore I wondered to myself - why does it feel like we're getting patches fast and furiously? Why do I hear and see people discussing patch burnout and talk myself about feeling totally overwhelmed by patch changes? If, as we've demonstrated, Mists of Pandaria is in fact more or less on par with or at best slightly ahead of previous expansions in terms of patch frequency (to go back to Wrath of the Lich King, patch 3.3 came out roughly a year after patch 3.0, although we then had a full year before Cataclysm) then why does it feel so much faster?
The fact is, every patch so far this expansion has moved the story forward in a way that has changed things. Patch 5.1 brought the Horde and Alliance in force to Krasarang Wilds, changing an entire zone and adding unfolding quests to several zones all over the continent. This lent a changed feel to the expansion - the first few months of patch 5.0 were months of discovery, of new lands and new ideas, and patch 5.1 brought the rest of Azeroth's problems crashing like a wave onto the shores of this new world we'd just begun to discover. Patch 5.2 upped the ante, showing that Pandaria itself had plenty of menace of its own with the legacy of the Thunder King, and added not only a new daily quest hub and a new raid but the Isle of Giants, showing us that Pandaria still had secrets we hadn't discovered yet. And Patch 5.3 gave us scenarios that exposed how events in Pandaria were reaching back to the old world, changing Azeroth in turn.
To be fair, this was something we saw to a degree in Cataclysm, with patch 4.1 and 4.2 bringing in new content (a phased Stranglethorn and quests in Zul'Gurub and Zul'Aman for 4.1 and the Molten Front dailies changing Hyjal in 4.2) but the Mists patches have been much more integrated into the story, and have felt more important for that reason. You could ignore the STV quests leading up to ZG and ZA and just run the dungeons over and over again, and the Molten Front just felt like pointless busywork when you could just raid the Firelands and be done with it. While I can see the way that the Cataclysm patches inspired and informed the design for Mists of Pandaria, the latter expansion has been far more successful in making each patch feel like chapters in an overall story.
Another aspect of the Mists patch cycle that helps create that feeling of rapid patch development is how each patch seems to experiment with the right way to tell a story via game mechanics. The changes between patch 5.0 and 5.1 in terms of the way daily quests work showed a series of progressive changes that (to my mind anyway) made the Operation: Shieldwall/Dominance Offensive daily quest grinds the best of the expansion to date. Patch 5.2's use of solo scenarios was another solid innovation, while patch 5.3 deliberately made the story as accessible as possible. Since these were significant changes in how content was accessed by players, and it happened in every patch to date, the sensation of rapid change is accelerated by the changes being both visible and meaningful.
Finally, the fact remains - Mists of Pandaria has had the most regular patch cycle. Both Cataclysm and Wrath before it had patches last irregular periods of time - Ulduar was barely out for three months before Trial of the Crusader dropped, but there was a six month gap between that patch and the Icecrown Citadel patch. Similarly, patch 4.1 and 4.2 were originally intended to be one patch, and their division led to a period of nearly six months between patch 4.2 and 4.3. Mists of Pandaria, by alternating its raid patches with patches focused on other content, has managed a solid three or four months between patches and that matters in terms of how frequent the patches feel.
I would ultimately argue that we're finally seeing a patch cycle that we should have been seeing all along. If anything, it's not at all too fast - other expansions were simply too sporadic.