'X' marks to indicate quality? The iconic mass-produced lager in Queensland, Australia, is XXXX (pronounced FourEx). On their label, they tell a story about how historically beers were marked with a number of Xs to indicate their quality, with three Xs being the best.

According to XXXX, the original brewer found that his beer was so good that he added a fourth X.

So what's the real story with the 'X' mark? I know there are other beers, such as Dos Equis, which incorporate an X in their name and branding. I have also heard of a number of Xs being used with whisky as a crude ABV indicator.
X's and K's actually derive from turn of the last century British brewing practices where the letters and how often they were repeated denoted the basic style and relative strength of the beer.  See more here.

The more in depth answer is that back then malting and kilning practices weren't as good so you generally just ended up with "Malt", which was kindof brownish but varied wildly.  So you just sort of made "Beer" out of it.  X Ales were table beer, made to be consumed relatively fresh, and K Ales were keeping beer made to be kept.  Hopping rates likely differed as well as starting strength.

I think at some point later X came to denote "mild" ale, which referred to a lower hopping rate and K/AK/etc came to refer to "bitter", but the blogger I linked to above knows more about that progression than I ever will so I may be totally wrong.