In a way ultrabooks will help bridge the gap between a netbook and a laptop. Many are 3lbs or less. I believe the 13" Samung 9 series is one of the lightest at around 2.4lbs - 2.5lbs. Therefore, ultrabooks are very portable.
Ultrabooks also use ULV (ultra low voltage) mobile Intel Core i3/i5/i7 CPUs. AMD is not in the picture yet, but they will enter when Trinity is released. Therefore, ultrabooks will have the CPU processing power that's equivalent to a laptop.
However, the price of ultrabooks are higher than many mainstream laptops; starting at about $1,000. Making things small, light and powerful is not cheap. Also, to be classified as an ultrabook, it must have a SSD drive instead of the typical HDD drive which increases the price (and decrease storage space). Additionally, for people who want to play games will have to rely on the Intel HD 3000 graphics core. Dedicated graphic cards takes up space and eats up power.