It's an instant-on appliance with a real web browser and real mail reader on a real screen. I guess that's the selling point right there. It's not a computer that requires booting and starting up system services and logging in. It's an appliance you can immediately turn on and start using.
That particular point doesn't seem like a huge win to me, as someone who never shuts down his laptop. I just open the lid and it's on, then close the lid and it's "off". The form factor/weight seems like the bigger win, at least for me. Although if you're going to carry around a Bluetooth keyboard to use it, like jwz apparently does, that seems like it eats away at the advantages there a bit?
When I do the closelid/openlid thing on my Debian laptop, it takes a good 10-20 seconds for my wifi to reacquire...and that's after I log in. Then I need to tickle Chrome or my mail client to sync up. I'd say maybe 30 seconds before I can see new email or load a web page.
In playing with the iPad, I've found it just comes up right away and I can see mail in 10 seconds. I can also scoop the iPad off the counter with a drink and some other items and plop on the couch. Not as easy to do with my laptop.