Academic definitions aside, there hybrids have to choose a very serious compromise due to the balance:
if you make the base light (surface pro case), you won't really have a laptop, instead a tablet which sits on your legs.
if you make the base heavy, you're making a relatively heavy laptop.
now, there is of course a spectrum of choice, but practically speaking, even they make a very light tablet part (say, 850 grams), with a "heavy base", you will add 550 grams at least (550 being an odd balance).
I'm absolutely happy to have a 1.4 kg hybrid, but it's not going to compete with the upcoming wave of ultrabooks (note that I'm somewhat skeptical it will be 1.4kg, I bet it will be a little more).
There has to be some trade offs? If they pull this off thou.. I'll line up for one.
The problem for me, under Windows 8.1, has been getting the machine to enter a true low power sleep state. If the machine enters the true low power state than it can work like a tablet, and if you use it lightly (say an hour a night) you can get a week's worth of use without recharging.
Unfortunately if any of the hardware components doesn't go to the low power state, the device will likely be dead or close to it if you don't use it for a day. Microsoft has a utility which tells you which component is keeping device from powering down, but doesn't tell you what software is causing this behavior. Windows 10 is supposed to improve this, but you can't turn off windows update so you lose one way to fight the battery drain.
One way you can fight this, besides plugging it in every night, is powering it down when you are finished, which isn't as bad as it seems because the device boots cold in about 5 seconds.