Bitcoin is much more powerful then Linden dollars, Egold etc. because it allows transactions online, anonymously, and has no central authority. No other payment method has ever had all three of these characteristics.
I can create an anonymous Second Life character at any time, linked only to my account info (an email - Mailinator solves that). I can build something in-game and sell it, creating money with no person bound to it. Or be given money in-game for a real-world purchase, leaving me with money that still has no connection with me. There's barely a central authority, as there's nothing stopping you from passing money around in-game, which would be the major barrier to illegal trading. And what is there is rather impotent, as everything can be done just as anonymously as in Bitcoin - they can get your IP address, but only if they're watching, and that's far from an identification. Just go to a coffee shop.
Bitcoin just makes it easier, and fault/attack-tolerant. It's been possible for quite a while. Though perhaps the most useful aspect to this is that transactions aren't revokable, where they are in nearly any other non-physical system.
Why single out Bitcoin? Because it's new?