Well with the 1st level thingy out of the way, barring some obscure feat combo, I would say it makes the game actually more balanced. Just one form available is a big hit on wildshape, since it forces the druid to choose between combat form and mobility form every level.
Is that too much for a flaw though? (compared to -1 AC
What can I say? Even just one wildshape form is still pretty damn powerfull once you take natural spell and you pick said shape well. Many druids I saw would be more than happy keeping just one shape all the time in wich they would specialize.
And you just made said shape immune to pierce magic concealment and AMF and whatnot. That could be argued to be actually an advantage.
Now -1 AC isn't as signigicant, but it's also something that's there all the time and can't be used for your advantage. It's hard to make a proper comparison. Personally I think it's a fair deal.