The responses I read can be broken down into three groups.
Group 1. People who haven't used Mac products all that long and went down the Panic! Doom! road. I am in this group. So was Marco himself. A lot of us freaked out because here is this guy (Marco, not me) who develops for iOS, uses a Mac, and has the respect of a lot of the Mac community and he's seemingly losing faith in the Mac platform.
Group 2. Long time Mac users who, while not resorting to panic, still pointed out that, yes, Apple has a lot of issues on the software front. In this group was Craig Hockenberry with his post, Death by a thousand cuts and Glenn Fleishman's list of The Software and Services Apple Needs to Fix. It's a long list of issues that any intermediate to advanced Mac user will find very familiar. Dr. Drang's Apple Leverage is also in this group.
Group 3. Long time Mac users who see this as yet another moment in a very long history of complaining about all the things Apple does wrong while still being the best computer company out there and continuing to get bigger and better. Gruber argues that Apple hasn't lost the Functional High Ground because no one else is picking it up and running with it. Daniel Jalkut, who's been following Apple since 1996 and actually worked for the company, weighed in with this post in which he lists a major issue for every year since he started blogging in 2005 and sums up the, "Hey. Everybody calm down," sentiment nicely:
Apple is clearly doomed. I think Apple is going to be okay.
So, maybe let's don't panic. I now think that the best course of action is the one Ashley Nelson-Hornstein advocates:
I’ll be justifiably concerned and worried if the same software quality issues are being discussed in 10.11 and iOS 9. Until then, I’m willing to give Apple the time necessary to let their plans propagate.
I’ll be checking back in another six months, during WWDC.
Marco since posted this about what the aftermath of his post was like, including being brought up in a piece on CNBC. In summary, it really sucked for him.