Changing blade flurry to minion only would have sufficed. They didn't need to double the mana cost on top of that. Koyuki mentioned something in his stream that I agreed with, it's actually a triple nerf if you count each point of casting cost increase as a separate nerf, which you really should. Look at how many cards have gone from overpowered to completely unplayed from a single increase to their casting cost?
Leeroy Jenkins - almost completely unplayed until Reno decks added him with Thaurissan.
Soulfire - completely unplayed
Flare - completely unplayed
Gadgetzan Auctioneer - almost completely unplayed until tomb pillager (and Thaurissan) brought miracle back
Sylvanas Windrunner - well she's still very playable, but she went from an auto-include in almost everything to only fitting in a few select decks
Blizzard/Frost Nova/Cone of Cold - iirc freeze mage completely died until Naxx brought mad scientist to the meta
Regarding the upcoming changes
Ironbeak Owl - Silence is still an incredibly powerful effect, but I suspect people needing a neutral silence will be more likely to use spellbreaker now.
Hunter's Mark - Despite the nerfs, I suspect hunters will be forced to run either Ironbeak or hunter's mark to punch through taunts. I think ironbeak being a beast will be enough to make hunters prefer it over spellbreaker, but double hunter's mark will probably be best
Big Game Hunter - After thinking about it, I think control and slower midrange decks will still want to run one. It's still removal with a body, and with the standard meta presumably being quite a bit slower + new cards from whispers, I'd expect to see quite a lot of fatties being played.
Knife Juggler - Probably still an auto-include in aggro decks. An unanswered juggler is still an assload of free damage
Leper Gnome - Probably not good enough for anything but suicide rush decks, and even they may not want it.
Arcane Golem - Unplayable shit now, maybe would have been playable as a 5/5.
Molten Giant - I think moltens are unplayable now. Not getting free giants until 5 health now is too risky for anyone but mage, but duplicate and echo are both rotating out.
Ancient of Lore - Druid probably still runs this, they just might pick healing a bit more often now.
Force of Nature - 6/6 for 5 wouldn't be very good because a neutral 5/6 for 5 already exists. 3 2/2s for 5 is trash.
Keeper of the Grove - Despite the nerf to the body, I think the sheer versatility of keeper makes it remain a 2 of in every single druid deck.