It's a combination of the two. This is one of the healthiest metagames we've had in a very long while. Every class is viable, which is something that's basically never happened in the history of the game. Aggro, midrange, and control are all fairly viable in one form or another. The non-interactive combo decks that do exist either aren't as degenerate as some of the older styles of combo decks or aren't as popular/good (although it took some time for people to realize that).
Rotation is an important part of this, for sure. But there also aren't too many reality-bending cards in Un'goro that are so busted that they degenerate the meta-game. We don't have any new cards that singlehandedly force the direction of the meta, like a Patches or Mysterious Challenger. Rockpool Hunter and Crackling Razormaw are the closest to being unacceptable but they still aren't on the level of Tunnel Trogg or Mad Scientist in terms of broken as hell early game.
And hey, multiple archetypes of Priest are viable. That's a ridiculous accomplishment.
That's a lot of good points and I agree.
The thing I have to say is the reason so many archetypes are viable now are
1) the absence of the cards rotated out, people still optimizing decks
Example, elemental shaman has incredible midrange and control value, but people are still experimenting. They only lost trigger and totem golem, which can be replaced with firefly and igneous, or that heal for three elemental. 4 mana 7/7 is still very good in aggro or midrange
Pirate wArriot lost nothing. That crab is worthless since pw is going face anyways.
Jade Druid only lost azure. That deck will be the premier anti control deck still,they also gained earthen scales for some heal.people are being bamboozled by aggro Druid. It's good but not that good. A maelstrom or lightning storm or devolve ruins that win condition.
2) people didn't have to consider old cards being rotated out before.
I had a lot more thoughts but I'm drunk and can't articulate on mobile
