One of the major points of criticism is that overusing Claymorphism and going overboard with colors and typography can give off a somewhat childish feel to the UI. Michał Malewicz acknowledged this drawback and gives a few suggestions on how to avoid that.
“The friendliness of those shapes, along with the 3d imagery creates a slightly child-like interface. To counter it, it’s good to use minimal typography, strong accent colors good contrast.”
— Michał Malewicz
If foresee a trend in classical Russian literature inspired user interfaces as a reaction. The Dostoyevsky style, dark and deep.
That's just your impression, though. The Windows XP Luna look felt childish because it was dog slow on XP-era machines compared to the classic theme, so it ended up being pretty disrespectful of its users. But a smooth-shaded high-relief button is arguably the best equivalent to that "Classic" look on our modern, graphically-accelerated, variable-DPI environments. It avoids the overly-detailed skeuomorphic look while still being immediately intuitive.
To mention some of the most awfully repulsive, having to interact with Travis and MailChimp are always sure to trigger a physical disgust response, and checking Sentry, I was greeted by a similarly awful cartoonish homepage. I can't not think of "cynical company that will sell you to the highest bidder if they can wants to be your BFF, they'll even give you candy!".