I've been playing a moon druid in OP, and am currently at level 3. There's no question that WS is very powerful at levels 2 and 3, and probably at 4 as well. But while it's definitely at or above the top end of the PC power curve for those levels, I don't think it's actually overpowered, and certainly not broken.
First, the inability to cast spells in beast form is a significant tradeoff. If you're the party's primary healer, as I was in most of my sessions, then choosing when to shift into and out of beast form is a big deal, as is the decision whether to burn your spell slots to heal yourself.
Second, the available beasts have low ACs. My experience was that the dire wolf's 37 hitpoints vanished pretty quickly when facing multiple opponents or one opponent with multiattack. So far, wild shape tanking is less impressive in practice than on paper.
Third, and I realize this is hugely subjective, I never had the sense that I was overshadowing other PCs of equal level, particularly the spellcasters. I don't recall having an equal level fighter in any of my parties, though, so caveat emptor.
Fourth, having only two transformations per short rest, with no increase until 20th level, is a significant limitation. This will depend a lot on your short rest metagame, and how much use you're making of WS out of combat. If you're heading into every fight with both transformations available, then WS is much more powerful than if you're doing 3-4 fights per short rest. Staying in beast form between fights to conserve transformations has serious tradeoffs if you try to RP the communication limitations.
Finally, I'm pretty sure that the moon druid will fall back to the standard power curve at level 5 and thereafter, but it'll be hard to say for sure until we get the Monster Manual and see what the full selection of CR 2+ beasts looks like. Obviously there's a huge bump at level 20 with unlimited transformations and almost no limits on spellcasting, but that's not an issue I ever expect to deal with, and it's not at all clear to me that it's out of line with some of the crazy stuff that other classes get at level 20.
Also, a DM who's concerned about this in a campaign can exercise some control by strictly enforcing the rule that you can only turn into a beast that you've seen. I would have no problem with a DM ruling that a druid who was raised in a standard western European setting can't turn into a lion, tiger or elephant, and that things like dire wolves and giant animals are off limits until you've actually seen them in the course of the campaign. (And wouldn't it be a great use of a druid's downtime to seek out new forms? You want to add Giant Boar to your WS options? Spend 10 days of downtime and make a Survival check.) And it looks like most of the higher CR beasts are dinosaurs...
All that being said, it doesn't take away from the fact that charging around as a dire wolf at level 2-3 has been a lot of fun - higher AC and hit points than a brown bear, easy access to advantage via pack tactics, and knocking enemies prone. If as a DM you genuinely see this as a problem, then limiting moon druids to CR 1/2 forms at levels 2-3 seems like something worth trying, but I'd recommend you give RAW a fair shake before going there.
