A lack of balance. Certain tobaccos dry the mouth, such as olor, a variety of filler tobacco grown in the Dominican Republic. In a seminar one year at the Las Vegas Big Smoke, Davidoff cigarmaker Hendrik Kelner passed out cigars made entirely of olor (save for the wrapper) to show how an out-of-balance smoke tastes. "It makes your mouth dry," he said, "and God made your mouth to work wet." A cigar containing too much olor, he explained, has a drying effect on the palate.
Olor triggers the salty region of the tongue, drying the mouth, while a tobacco such as Dominican San Vicente stimulates the production of saliva. Many cigarmakers strive to create a balanced, complex cigar made with a variety of tobaccos, which stimulate different areas of the tongue.
Of course, even smoking the most balanced of blends is a thirsty business, and there's no shame in quenching that thirst, so long as you make the proper choice.