Andrew Revkin, who writes the New York Times' Dot Earth blog:

This is an important step on two fronts — sustaining domestic momentum away from coal in electricity generation and providing a fresh signal to other countries that the United States is committed to cutting its carbon footprint.

Brad Plumer, Vox:

A bunch of media outlets are referring to this as "Obama's climate plan." But that's not quite right. More precisely, this rule is just one piece of a much broader Obama agenda to reduce US greenhouse-gas emissions over the next decade. The Clean Power Plan is certainly a significant component, but it isn't even expected to account for a majority of the cuts Obama's envisioning. So keep an eye on all those other rules and policies, as well.

Michael Levi, Senior Fellow for Energy and the Environment at the Council on foreign Relations:

Politics has greatly constrained the realm of the possible for emissions cutting policy. A fundamental shift in U.S. politics could in principle yield something substantially better – but that isn’t the universe we’re living in. For the time being, the principal alternatives to the Clean Power Plan as it stands are inaction; a different set of EPA regulations that’s far less flexible (and hence less economically sound) or far weaker; or, potentially, large subsidies to a range of zero-carbon energy generators. The Clean Power Plan is a vastly superior way forward.

Michael Grunwald, Politico:

by the end of this year, according to Bloomberg New Energy Finance, the power sector’s emissions will already be down 15.4 percent from 2005 levels — about half the anticipated reductions in just a decade, and before the plan goes into effect. In other words, even under the strengthened plan, the rate of decarbonization is expected to slow over the next 15 years. What, did you think the strongest action ever taken to combat climate change would actually accelerate the nation’s efforts to combat climate change?

The final rule will also delay the first deadline for states to meet interim targets from 2020 to 2022, a significant walkback in a plan that Obama, cueing the Times, called “the biggest, most important step we’ve taken to combat climate change.”

If you’re really ranking them, the Clean Power Plan is at best the fourth-strongest action that Obama has taken to combat climate change, behind his much-maligned 2009 stimulus package, which poured $90 billion into clean energy and jump-started a green revolution; his dramatic increases in fuel-efficiency standards for cars and trucks, which should reduce our oil consumption by 2 million barrels per day; and his crackdown on mercury and other air pollutants, which has helped inspire utilities to retire 200 coal-fired power plants in just five years. The new carbon regulations should help prevent backsliding, and they should provide a talking point for U.S. negotiators at the global climate talks in Paris, but the 2030 goals would not seem overly ambitious even without new limits on carbon.

Kevin Drum of Mother Jones responds:

This is a little unfair in two ways. First, the 15 percent reduction of the past decade was the low-hanging fruit. The initial cuts are always the easiest. The next 15 percent will be harder, and mandating that it happen at about the same rate is more stringent than it sounds.

Second, the decrease over the last decade happened mostly because gas-fired plants became cheaper than coal thanks to the boom in natural gas fracking. That's a one-time deal, and there's no guarantee that something similar will drive further decreases. Having a mandate in place forces it to happen regardless of future events in the energy market.

David Graham, The Atlantic:

In brief, the Clean Power Plan puts limits on carbon pollution from power plants, mandating a 32-percent reduction by 2030, though based on 2005 levels, setting state-by-state standards for reduction. The rule is expected to lead to the closure of many coal-fired plants and prevent new ones from opening. The regulation was first proposed last year, and, after the EPA considered public comments, the final rule was released Monday. Experts described the rule as historic.

“They’re the most important regulations on climate change ever issued by the U.S.,” said Michael Gerrard, director of the Sabin Center for Climate Change Law at Columbia Law School.

Gerrard said while the rule’s impact would be important stateside, it was at least as important because of the role it will play in the global negotiations in Paris. Although China is now the world’s largest emitter of greenhouse gases, the U.S. has historically been larger, and American action is seen as essential to urging other countries to move on reductions. Following Obama’s proposal in 2014, China announced new emissions targets. But international progress is fragile, and attempts at marshaling a global response to climate change have repeatedly foundered.

“If these rules were to crash and burn before the Paris conference, that would likely have disastrous effects there,” Gerrard said.

 The Economist's Democracy in America blog:

Even if domestic leadership on the EPA’s proposals remains uncertain, the plan suggests America wishes to occupy a bigger role at the climate negotiations in Paris this December. Ethan Zindler, from Bloomberg New Energy Finance, says the measures finally “sync up international promises with domestic policies.” America’s emissions-reduction deal with China late last year now has more bite, for example. And any indication that America is more open to multilateral negotiations is welcome news elsewhere in the world, even if many allies had hoped for a more ambitious environmental agenda.