Second, I think the blog post is useful because it shows what's wrong with some (fortunately very small) part of the Scala ecosystem, and because it points to a way to fix it.
Here's a quick recap: The author tries to add arbitrary operations to Scala's Seq abstraction without changing its source code and wants them to work also on arrays (which are plain old Java arrays), without any extra work. Arrays in Java support: length, index, and update; that's it. There is as far as I know no language in existence that allows the precise thing the author wants to achieve. And there are many variations, such as adding only to Seq or only to Array that would be really easy in Scala but still impossible in most other mainstream languages. The author then throws all the machinery he can think of at the problem to still achieve the same non-result. Well, tough luck. He might have hit a thing that's simply impossible to do in a generic way, given the tools we currently have. In fact, I have not checked whether there would be a way to achieve the result that he wants because that's beside the point. There are always limits to a generic formulation that will force you at some point to treat things on a case by case basis.
The problem is that, in trying to achieve his impossible goal, the author (mis-)uses a lot of the most powerful features of Scala, and concludes that Scala is simply too complex for helping him achieve the result. I believe he wrote this blog post to prompt the maintainers of Scala to add even more power to the language so that he can achieve his goal (the only other motivation I can think of is that he's trying to actively damage the ecosystem he writes code in, but that would make no sense to me).
My response will probably not please him. I think that we need to take away sharp knifes from people who have a tendency to cut themselves. I was always proud that in Scala you could do in a library where in other languages you had to change the compiler. Inevitably, some of this is cutting edge stuff. We have tried many times to clarify the boundaries, for instance when I defined the Scala levels
http://www.scala-lang.org/node/8610
But we can't prevent a developer who prides himself to "stroll right through level L3" to get hurt.
So, I believe here is what we need do: Truly advanced, and dangerously powerful, features such as implicit conversions and higher-kinded types will in the future be enabled only under a special compiler flag. The flag will come with documentation that if you enable it, you take the responsibility. Even with the flag disabled, Scala will be a more powerful language than any of the alternatives I can think of. And enabling the flag to do advanced stuff is in any case much easier than hacking your own compiler.
I would be interested to read your comments on this proposal.