In an earlier post I said the same thing about using my 13 inch MBA most of the time in preference to my iPad and my MBP. It's a fact, once you have an MBA you will mostly ignore the other devices...
That happened to me as well. Since I've got the 11" MBA, the 2009 13" MBP went to my father, (his 2007 macbook stopped working, but it went through 4 batteries, so I guess it has been working rather hard), my iPad 1 was sold and my MBA 13" 2010 is just hanging around on my desk at work.
The MBA 11" 2011 is quite a capable machine. And over the weekend, I was able to get 7h13min of battery life by using it as just a very expensive typewriter.
Compared to all the other Apple laptops, the MBA 11" is just as capable for every day tasks, it is smaller than all other laptops from Apple and the only sacrifice is in battery life.
When compared to the iPad, well, where do I need to start? full featured, with a keyboard, with OS-X instead of iOS, 40% heavier as a bare iPad, but a bit lighter than an iPad with a bluetooth keyboard.
So there rare enough reasons to buy a MBA over the other products in the Apple line-up.
As to the why people are more likely to react to a negative post, you can make quite a philosophical and/or psychological analysis of the groups of posters that frequent Macrumors.
But the easiest way is to just conclude that people are more prone to gossiping than to giving positive feedback. For most people it's easier to say what doesn't work for them than to say why it does work so well for them.
You have people that don't have an MBA, don't want a MBA, don't like the thing and can't see how you can live with "so little computing power" "so little battery life" and/or why you would want to shave of 30% or even 50% of it's weight. For them it's easy, they just write of the MBA in their posts.
There are people that have had a MBA and it didn't work for them, they just can't get it to work for them, so why would it work for others?
There are people who can't afford the MBA (it is quite expensive after all) and just dislike everybody who can (it's petty, but quite common).
And there are people who do have an MBA and are quite happy with it.
So getting negative feedback can be quite a lot easier than it is to getting positive, which can be a reason.