Vodka = alcohol + water, and nothing else.
Actually, there are varying qualities of alcohol and water.
Good vodka is smooth and easy on the taste buds (usually comes from careful filtering--I prefer 4 times--and good quality ingredients).
I also find that another important quality in vodka is how agreeable it is phyically (i.e. even a small amount of poor quality vodka can have negative consequences for tummy disturbance and/or headache).