Most brewers talking about dextrose are referring to corn or table sugar. It's a very simple sugar that the yeast will consume first and it does not seem to affect flavor. Sugars like honey, brown and molasses have other types of sugars and aromatics that affect flavor. If you taste regular sugar it doesn't have too much of a "flavor," it's just sweet. But when you taste honey or brown sugar you get a flavor, mainly from other compounds. Also, different types of sugar have different degrees of fermentability, if it is less fermentable it will lend more flavor. Something like brown sugar has unprocessed sugars and molasses in it so it gives the beer more flavors.