Regardless of quibbles over stat allocations or playstyle or the pro's and con's of barkskin, I think the main point I originally wanted to make is that I think the moon druid is fine - as along as it isn't taking away from other players' enjoyment of the game. If I had the same experience as rjfTrebor (3rd level druid faces down a cyclops, overshadowing the pure melee builds), I'd be a lot more concerned.
But at my table, that druid would have 11 AC (no barkskin), and the cyclops is hitting him on a 2. His wildshape is gone in 1 round, probably with some roll-over damage on his main character. See my anecdote about the owlbear, which was only CR 3. That druid only lasted a few rounds. I think this is where it's important that the DM adjusts according to the specific circumstance. If the druid's in wildshape form, he should be the priority target for the cyclops if you think the party is having a hard time of it (you can still dish out massive damage on the party, without arbitrarily killing the other PCs). If, on the other hand, they're having it too easy... smack out one of the other melee types to make them sweat, then switch to the druid. Even if they handle the cyclops easily, thanks to the druid burning 2 wildshapes to soak damage... good luck with the next 2-3 encounters before you allow a rest. Or 4 encounters. Or however many you choose to push their way until their eyes begin to twitch.
If the other players are grateful that the druid is soaking the cyclops' beats each round (rather than killing their own characters), then it's not a problem. If the druid is also doing the majority of the damage, then I'd question the barbarian or fighter build. A frenzying 3rd level berserker should be attacking twice per round for higher attack and more damage than the bear, with resistance to physical damage and higher HP. A 3rd level fighter should have much higher AC and attack bonuses, slightly inferior damage, but with nifty tricks using their superiority dice. So, the druid has more hp? So what. He's getting attacked more by the cyclops. Damage soak has nothing to do with coolness and player empowerment. It's about the amount of Awesome they're dishing out on their own turn. Both the fighter and the barbarian should be beating out a boring ol' bear, even at 3rd level.
As usual, I think it's just another point towards the increased role of the DM in 5e. However, bounded accuracy and the more brief/lethal nature of 5e fights makes OP builds less of a problem than recent editions, I think. In 3e, you could get some really broken builds that had numbers that just couldn't be matched. Huge spell DCs, unhittable AC, massive damage combos, Christmas Tree magic item sets. In 4e, you had sort of a different problem: the builds weren't as broken, but you also didn't have as many tools to challenge the PCs. No save-or-die, and monsters outside the +/- 5 CR range were basically pointless due to the way the math worked. In 5e, there's almost always a solution to make the player sweat.
Usually, it's an intellect devourer. Intellect devourers solve nearly anything.