Here's some further reading:
Note that Darin's followup message included this:
"I do agree however we want to keep Pepper APIs in sync with existing Web APIs. That will obviously be a challenge if they are not one and the same, but I don't think it is impossible or even that difficult to achieve with them being separate APIs."
This has already failed. Web Workers cannot render even 2D content asynchronously today, while Pepper can (which advantages plugins and NaCl over ordinary Web content). This is exactly the problem that roc and smfr were trying to avoid.
Also, the argument that "it's an existing plugin, you don't want them to rewrite their code" doesn't hold water. It's an entirely new set of plugin APIs. Adobe has to rewrite its code anyway. It's just that the other browser vendors wanted the Web as a whole to benefit from this, not just Adobe.