I don't think many people realized this but iPad 3 is actually heavier than iPad 2, and many might still remember how "light" the iPad 2 was compared to iPad 1. iPad 3 is somewhere in between the iPad 1 and 2 weights.
But I think the ideal weight for a tablet is around 300 grams, which half of what most 10" tablets have today. This is probably the most important reason why the "main iPad" will eventually become iPad Mini. 600 grams is too heavy for extended and comfortable use.
i'm curious if there's any reasoning to 300 grams. I don't have an iPad, but i've used one, and it seemed fine to me - i guess over a long period it could become uncomfortable.
I read my iPad2 in bed pretty frequently, it's just heavy enough that resting the weight of the device on a single edge on your chest will become unpleasant rather quickly.
As an extra datapoint, a Kindle (170 grams) is trivial to lightly with a few fingers. It's no substitute for a tablet, but when all I want to do is read a book I love it so much more than any heavier, more capable device.
But I think the ideal weight for a tablet is around 300 grams, which half of what most 10" tablets have today. This is probably the most important reason why the "main iPad" will eventually become iPad Mini. 600 grams is too heavy for extended and comfortable use.