Both are important. You really need both to be compelling.
Why? Because Steve Jobs says so? Again, there's utility to having something ultra-light. There's very little practical advantage to having something ultra-thin. If anything, I think it's safe to say that the latest tablets have reached a point of diminishing returns on thinness.
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.
If you're trying to hold the device with one hand, anchored with your thumb while trying to keep it on the bezel, then it's way more comfortable if your thumb doesn't have to be butt up against your index finger. The human thumb is designed to be opposable. When you grab something thick (at least 1"), you can see your thumb turn over to oppose your fingers. Try to do the same thing with something really thin. Your thumb doesn't turn over, and you end up simply bracing the object against the side of your thumb. Your thumb has little leverage this way, and it's uncomfortable as hell.
With that said, I'd love them to be half the weight. But who wouldn't?
I'd gladly trade thinness for weight any day of the week.
MS can't ship Touchpad size/weight tablets though with Win8 and expect much in the way of sales.
I can guarantee that Microsoft won't ship Touchpad size/weight tablets.