This is a question of been pondering for a while. I think Andy Bourland hits the head in his renewed column on ClickZ.
Moving from "free" to "fee" is risky. If you offer content that's simply "nice to have" rather than "vital to have," chances are your audience won't pay you a dime. If you have a bunch of competitors offering similar content for free, you're not likely to succeed in charging for what your readers can get elsewhere for nothing. If your audience is ill-defined or if the solutions you offer are only a modest improvement over the status quo, they won't open their wallets, either.
Those offering superior to none content will thrive in this model. Those offering information that's a commodity will fail. Regardless I think just as it was foolish to rely on just ad revenue to keep a site up and running, it would be foolish to rely only on paid subscriptions.
I've got an idea buzzing in my head (see last few entries) about introducing blogging as a source of income. Stay tuned ....