It's a tribute to Rails' genius that so many people view it as a threat. You still see people who seem genuinely afraid to let go of their massive, overly complex frameworks, or optimizing every little bit of their PHP app. People afraid of relinquishing the slightest bit of control for convention.
What I actually see, when I hear developers decrying this stuff, is geeks afraid that they're going to lose their Genius cards now that there's an easier way.
The fact is, Rails doesn't reduce the amount of hard thinking required to write a reasonably complicated web app, it reduces the amount of boring boiler plate code. For heavy lifting Rails still gives you the tools you need, there's a reason Rails Metal and find_by_sql exist. There's a reason most of the core parts of Rails are interchangeable (esp. with the upcoming Rails 3). Solving any worthwhile problem generally requires real work. Rails doesn't change this.
The brilliance that Rails brings to the table is balance. Many of the design decisions of Rails just hit the sweet spot for a large category of apps, and remove so many of the pain points in development.