Isn't your 12 inch notebook able to be used just as easily on the treadmill? If not, why not? And it seems like video podcasts while ironing would be better with the 12 inch because you can simply set it down on any surface and set the angle of the screen to whatever angle offers the best viewing.
There are treadmills with enough space to hold a 12 inch laptop? Picture, please!
As for videos while ironing, iPad with smart cover or similar stand takes up about 2x9 square foot of surface area. A 12 inch laptop takes... I'm not sure, something like 9x11? That's a lot more surface area you need to find to place a laptop. So no, not any surface will work to set down a laptop.
in what way are they better than the vast amount of great news websites?
You have an iPad, right? Download Zite and Flipboard and see for yourself. They are both free.
the iPad weighs 1.44 pounds and the Macbook Air weighs 2.38 pounds. I believe that's not a meaningful difference in terms of how long an able-bodied person would feel comfortable holding it for.
Other people have addressed this already, but I'd like to point out -- you know that outside pocket on many bags for holding magazines and newspapers? iPad fits great in those pockets. Easy in, easy out, much faster than having to open up the main compartment of your bag to get out your laptop. Maybe the 11 inch Air might fit in *some* of those pockets, but not as many as the iPad.
The main use I have for my iPad is to read -- both the web and ebooks. For years before the iPad came out, I would be reading something on my laptop and wish there were some way to detach the keyboard and just hold the screen so I could read. The keyboard would take a lot of space on my lap and there was just no way to pppsiton the screen at a comfortable distance and angle without the keyboard getting in the way. The iPad is the perfect reading device for me, and everything else it does is a bonus. But it's also my calendar and address book, todo list, game machine, video player, I use it to text people, and in a pinch I can use Pages to get some work done on the subway. And before you say an Air will work as well or better, see above about fitting in outside pockets. When the train rolls into my stop, I just sleep my iPad and slip into an outside pocket. It's just a lot more quicker move than putting away a laptop.
Of course, an iPad isn't for everyone, so it's perfectly fine if you feel it isn't for you. I hope this discussion is giving you some sense of why it is for other people.