iPad will spawn a category of lightweight tablet computing devices that will slay the market for netbooks, IMHO.
Apple must stay ahead of these new tablets to maintain iPad's lead. New developments can include a non-tethered OS, sub pound weight, decrease in thickness, a smaller (more portable) screen and bezel, higher screen resolution, handwriting recognition, a front-facing video camera, and more storage and faster CPUs.
Sight unseen, iPad's weight, size, and user interface are already known (1.5 lbs entry level, 9.56 in x 7.47 in x 0.5 in, iPhone OS).
iPad's weight and size will be the biggest issues IMHO. 1.5 lbs is light by netbook standards, but not handheld standards (eg, 0.3 lbs iPhone, 0.7 lbs Nintendo DSi XL, 0.9 lbs Canon Powershot G11). Tabletop use will be common I suspect (eg, 1). And iPad's screen size and bezel can shrink without decreasing (and perhaps increasing) usability.
Bob Iger, president and CEO of Walt Disney, called iPad a "game changer," and I agree, but vigilance for Apple is in order.