I literally just came on here to complain about the spacebar too and saw this thread.
I absolutely love everything about my new 13" 2011 mba, except this spacebar. Like you said, it is really loud, but even more than that, it just doesn't feel right when I type. There is almost not enough resistance and "bounce-back" if that's the right choice of words, so it seems like it takes too long for the space bar to return to it's original position after hitting it. I have a hard time typing as quickly as normal because the spacebar feels "mushy." And it's really loud.
I previously had the 2010 11" mba, and had no problems with the keyboard whatsoever. I'm going to check out some models at the store and see if mine is defective or if they are all like this.