Wow, a thread in which everyone is making intelligent arguments, even while slightly disagreeing with each other. Such a rare phenomenon! I just have to join in ... 🙂
I agree with many of the points here, but not quite the one about Chickening. I do agree that it is slightly OP, but I think that is a good thing for the game. Many long-time Hearthstone players are beginning to complain that the lack of new removal spells in recent expansions, especially spells with AOE (area-of-effect), have made the game far too focused on minion combat. If you lose the board early it becomes very hard to come back. They want Blizzard to print new efficient removal spells because that enables a completely different play style (called "control") where you voluntarily relinquish the board, try to remove what the opponent plays, and come back in the late game with heavy hitters.
Look at it from the other perspective as well. Imagine this plant start with Chickening removed or nerfed. Turn 1 double mushroom, turn 2 another mushroom, turn 3 double mushroom and mushroom booster. Game over on turn 3. Chickening is what makes decks like this risky and keeps them in check. Otherwise everyone would run the mushroom combo.
Anyway, it's easy to play around Chickening. Avoid overextending with 2-health minions. Play a bigger guy on turn 3 (or don't play anything at all). That's it.
TL;DR: I'd like Chickening to stay because it enables alternative play styles for creative deck builders.