When it comes to tablets, thinness is only ancillarily related to the more important criterion--weight. If I had to choose, I'd rather have something lighter than thinner. In fact, the thinness of the current crop of tablets, Galaxy Tab 10.1 and iPad 2, make them more difficult to hold IMO.
Both are important. You really need both to be compelling. Although I disagree that the thinness of the current crop of tablets makes them difficult to hold. I hold an iPad2 regularly and have held a GT10.1. Both feel great in my hands. With that said, I'd love them to be half the weight. But who wouldn't?
MS can't ship Touchpad size/weight tablets though with Win8 and expect much in the way of sales.