It really is all about the initial design not the language. ASP.NET just makes it too easy to go with bad design and library choices, not to mention the tons of examples of bad design that newbies might accidentally follow at the beginning.
Explain? Example? I don't see how this could possibly be true for .net but not for Rails or Python or PHP or whatever.
WebForms in ASP.Net are pretty much an attempt to take the developer experience of Visual Basic and make it work on the web - arguably with mixed results.
I really don't think there is anything wrong with .Net as a platform - but things like WebForms can be rather disturbing if not used with care.
I'm a huge fan of .Net (in fact my startup runs on it) but I absolutely hate the 'magic' that WebForms does for you.
However, Joel Spolsky is a tool. Everyone I know and respect thinks he's a tool. He is a prime example of being successful because of being lucky, rather than being good. Do not quote Joel, he can't form a valid argument for a topic without circling back on himself to save his life.
And Copilot wasn't even envisaged or created by Joel, but was rather a project created by some interns.
FogCreek doesn't innovate (they have a bug tracker and a remote support application based on VNC). Joel just happened to be one of the first to blog constantly about technology. He doesn't even allow comments on his posts because he doesn't want to have to deal with people contradicting him (not that he doesn't do an excellent job at this himself).
There are so many great developers and businesses out there that you could use as an example to model your business and software on. Use them, not Joel. Let Joel's business and opinions fade away as other smart, innovative companies run rings around his ageing product suite.
Besides, this is a perfect example of him contradicting himself about how taking VC is a bad idea.
Besides, do you think they are profitable yet? How do you think they will get there without heading down the Expert's Exchange track? And let's not forget how much chaff is now being added every day to these sites, it now seems to be impossible to ask a difficult question because only relative newbies seem to be the only ones answering questions.
http://www.joelonsoftware.com/items/2006/09/01b.html
So I would say that he's actually got a pretty interesting perspective on choosing technology platforms for web applications.
His software runs on a compiler that has had 2 months work put into it and worse, it compiles to either VBScript or PHP.
It's probably the most counter-intuitive solution ever implemented to solve the problem of needing a web-based application to run on both Windows and Linux.