"Open type system" in this case means "the ability to make business logic and pieces of data model part of the type system."

So if I'm writing, say, an XML parser, the pieces of it from a defined XSD can now be part of the type system. If you try to use a tag which is typed incorrectly (i.e. isn't part of the XSD) you'll get a compiler warning.

This is, in its core, a language that's really meant to be used for scripting in a limited domain enterprise context. Sure, that means it's not as "pure" and "ideological" as some languages, but it's something that's useful in the business sense.

I can imagine my phone ringing and saying,

"Hey, Kevin, I just noticed you're headed to The Pub. Um, it might not be a good idea to be drinking right after that breakup. Just sayin'..."

Brin makes an argument that not only are we going in this direction, but that this direction is inherently reasonable. I'm not sure I agree with all of his claims about using "public shame" to help shape a more harmonious society, but it's still worth the read.

More about his book here.