Walt Mossberg nails it in his Wall Street Journal piece:

If people see the iPad mainly as an extra device to carry around, it will likely have limited appeal. If, however, they see it as a way to replace heavier, bulkier computers much of the time—for Web surfing, email, social-networking, video- and photo-viewing, gaming, music and even some light content creation—it could be a game changer the way Apple’s iPhone has been.

And by “heavier, bulkier computers” he means “laptop.”