See How to Disagree, by PG: http://www.paulgraham.com/disagree.html
Kent and his circle of friends were refactoring before there was a word for it; when XP and all its many ancillary methods were just called "best practices".
Not sure I've ever seen him publish something like this before. This is part of the problem with dismissing people over the Internet without really knowing their backgrounds. It is very easy to make a fool of yourself.
I think he treated Joel fairly. He was complimentary to Joel's efforts in writing but simply told him that he doesn't really know him and should be more careful with his opinions.
Let's assume that Joel is neither stupid nor malicious. Therefore he is having problems that presumably others are having. In this case it is useful to actually set out the nature of Joel's error, rather than simply saying "Joel is a dolt". Non-malicious criticism can often be helpful in letting you explore why people misunderstand you.
Of course my assumptions could be wrong - he could believe that Joel is being malicious.
FWIW, I think this is the post being responded to: http://www.joelonsoftware.com/items/2009/01/31.html
1. "Joel is neither stupid nor malicious" - Stupid + Malicious != AllThereCouldEverBe... Pompous and AttentionCraving are missing for example. Others are too...
0. I _partly_ dislike both stands on software dev. methodology (Kent's and Joel's) so don't take this a fanboysm. I respect Kent's life work more though.
More info here: http://en.wikipedia.org/wiki/Kent_Beck
More importantly, this whole "how to disagree" thing is about extending courtesy. Joel Spolsky dissing Kent Beck is like a four-year-old piddling on a real pioneer's foot. Beck did Spolsky more courtesy than Spolsky deserved just by responding at all.
A lot of people on the Internet don't seem to realize that when one person disrupts another's day with attacks or what have you, that other person might have other things going on that are more important to them than responding to the Internet crap, or which require so much time that no time remains for responding to Internet crap.
The subtext in "How To Disagree" is "you have to show me respect if you want to debate something with me." That's perfectly reasonable. But starting a fight with somebody out of the blue isn't about rational, adult disagreement.
Besides, dude, he's responding to something somebody said. It's not his fault you know about it. He didn't publicize it to you, and for you to judge him based on his failure to provide you with context, when it's you going to his site, that makes no sense at all. Be mad at Hacker News for linking to something you don't know about, or be mad at yourself for clicking links automatically and wasting your own time. I mean there's no logic in holding that against Beck at all.