Why is are the keys on my MacBook Air so stiff? I've bought an 11" MacBook Air a month ago. The 9 came out from the keyboard after few hours and I couldn't believe it. They gave me a new MacBook Air. One month later I'm using it and today the T is too strong to press and I can hardly type it. 

I had another Mac before, and it lasted for years and I had no issues with the keyboard. This laptop never hit the floor; nothing's happened to it. 

I can't believe I have to come back to the Apple Store and tell them the keyboard is broken again. What do you think about it? Is a line defect?
I doubt it, it's the first I've heard of any keyboard issues on any MacBook that I recall—the keyboards are traditionally highly regarded for quality.

I think sadly you've probably just had some bad luck. No matter how well designed it is, something as complicated and mass-produced as a MacBook Air is going to have some bad units.

You may want to try blowing some compressed air under that key though, it's possible that something is physically stuck under there, impeding the movement. Someone (ahem, possibly me) once ate a poppyseed bagel near my keyboard and a loose poppyseed caused some issues for a while before it got knocked loose.