[showerst]

The biggest criticisms I've seen so far revolve around two things:

1) Many existing polluters can't afford much of a limit, but we don't want them to go bankrupt, so every proposal offers tons of 'free' allowances, which will inevitably be allocated more by congressional pull than metrics. It also favors existing polluters and discourages new entrants, even though they may be cleaner than the existing market.

In Europe these free credits (and their unpredictability) caused the value of carbon prices to fluctuate wildly, which undermined parts of the program

2) <This criticism is stupid to me, but it gets around> - The credits will just turn into one more big derivatives market, and be engineered to obfuscate real polluting by clever gaming of the trading system.

These are definitely fixable issues, but it's reasonable to be worried that implementation may be so complicated that it has nasty unforeseen consequences, since setting up a new market and regulating it well is rarely easy.