"Powerful web applications that formerly might have taken weeks or months to develop can be produced in a matter of days."

This is true if you are already an experienced web developer. You already know the problem space well and have an intuition about good solutions and approaches. Someone handing you Rails as a tool can certainly speed up the development time.

This is also only true if the application is trivial; you can only create a simple "first cut" of a product in a matter of days.

"Ruby on Rails is a breakthrough in lowering the barriers of entry to programming."

I think Ruby is a great first programming language; it has a cleaner, more consistent syntax than some other languages with fewer gotchas and edge cases (I'm looking at you, JavaScript!).

Rails, on the other hand, has a lot of magic, so that if it is your first programming experience, you will probably approach programming more in terms of memorizing and repeating the magical incantations instead of having an understanding of how the system works and practice in designing and crafting good systems of your own. So in that sense, Rails (and the state of web programming in general) may make it harder for people to get really good at programming.

Yes, the barrier to entry to writing web applications is significantly lowered by Rails; but it's not meant to be a replacement for experience. Rails is supremely helpful, but not explicitly designed for beginners.