AsQuote from Malfurious >>
Druid could ramp quickly before and drop a big board. They also could gain a ton of armor. A number of armor-gaining cards got cycled out, so they put in Earthen Scales for Druid to be able to gain some armor.
The actual problem now is that it is much harder to clear a big board than before. Druid could drop just as much on the board before and gain just as much armor too. And with overgrowth at 4 mana, and nature’s studies and guidance to discover, they could ramp just as fast if not faster.
There are still ways to take out big minions, but not as easily as at the end of the last expansion, where you could build a board every turn and it would be completely wiped the next.
That armour gain they had was tied to summoning a random 8 drop, now they summon equivalent 8 drop in stats and can gain more than 8 armour, yeah sure it does take 2 cards instead of 1, but they also have 2 more copies of cards that can fetch other cards with Aquatic Form. Before rotation if the 8 drop didn't have taunt, or at least rush you would be losing a lot more health than the current guarantee of a large taunt minion provides. Aquatic Form also means the Druid can ramp early a lot more consistently and generally tutor key cards easier than before, on top of the Nourish buff, which although slower than Overgrowth the flexibility of drawing cards is well worth the extra 1 mana in a lot of cases.
As far as it's harder to clear board, well yes classes have lost a bunch of removal options with rotation, but even the classes that do have suitable removal for Druid boards (Priest in particular has multiple) the issue isn't not being able to clear boards, it's not being able to out resource a Druid on 20 mana and with Kazakusan (you can actually exhaust every card in Druid's deck, but Kazakusan gives it either too much burst, or resources).
Also for some classes it's actually even way easier to clear boards, such as Pirate Rogue with Plague Scientist and Pufferfist you generally only have to clear 1 taunt vs Druid anyway, because your deck should be able to kill them before they can play multiple or be designed with some kind of OTK/ burst to go over the top, any other type of deck isn't going to have a shot vs Druid.