Good Freakonomics post on why there are so many new competing brands of vodkas compared to other liquors. The consensus among the comments is that it's about brand and marketing vs. taste, since vodka is supposed to be as tasteless as possible.
Which makes for an interesting question for a marketing class somewhere: how do you differentiate a product that is supposed to be, well... perfectly un-different.
The answer, of course, is brand. When Absolut began its wonderful print campaign, they were in the relative basement of vodkas, to say nothing about booze preference in general. Within a couple years of that campaign's launch, they had not only increased their own sales immensely, but had increased the marketshare of vodkas across the board by 30%. Best of both worlds: they grew the pie and their piece of it.
It makes for a good way of starting one part of a branding brainstorm conversation. How would we brand this product if there was abolutely (ha ha) no product or service difference between us and the other guys? What role in customers' stories do we want our stuff to play? What kind of prop is it? How does it fit into their lives? What does our brand taste like?