In the Scotch world, single malt drinkers tend to look down their noses at blended Scotch drinkers.  Is it warranted?

Production

All malt whisky is created the same (and grain whisky with a few minor alterations).

Soak your grains in water until they start to sprout.
Toast grains over the span of several days to halt the conversion of starch to fermentable sugar. (Not done for grain whisky)
Grind malt into grist and steep in hot water.  The starches are converted to fermentable sugars in the water.
Add yeast and allow wort to ferment.
Separate solids from wash and distill (generally twice).
Stick it in a cask and let it ferment for at least three years.

This is the point at which the process changes.

Single Malt

At some point in the aging process, a cask of whisky will be deemed acceptable to bottling.  Casks of varying ages will be blended together to create a batch of single malt Scotch.  The single malt refers to the fact that the whisky must be made with 100% malted barley and is only blended with other whisky from the same distillery.  On a caveat, there is such a thing as single cask Scotch (which is incredibly rare, and more than you can afford), which is a bottle of Scotch that came from a single cask and wasn’t blended with anything else.

Blended

After a certain amount of time goes by, the distilleries will sell off some of their casks to blended scotch companies.  Each blended Scotch company will purchase enormous amounts of whisky from dozens of different distilleries, but a majority of their purchases will be grain whisky – not malt whisky.  That’s because 80% of blended Scotch is grain whisky, and the 20% that is malt whisky is a blend of up to 20 (but typically less than 15) single malts.

So What Does That Mean Regarding Flavor?

I’m glad you asked!  First of all, the grain whiskey that is used in the blending process has a higher alcohol content and is nearly flavorless so this has no effect on the flavor profile of blended whisky.  So the difference in flavor is that each distillery produces their own distinct flavor of Scotch and they put a great deal of effort into producing a quality, unique product that stands apart.  Meanwhile the folks making the blended scotches are spending just as much effort to create a quality product that focuses on consistency and moderate flavor as opposed to uniqueness.

You can think of it like mom-and-pop restaurants vs. franchises.  Some people would rather go to the franchise that they KNOW is above average no matter where in the world they are, while some people would rather try the mom-and-pop restaurant and run the risk of truly awful food if there’s a chance that it might be AMAZING food.  With blended scotch, you’ll always get a consistent flavor that isn’t too over the top.  With a single malt, it’s hit or miss.  Luckily, if you come across that single malt that makes your mouth salivate just thinking about it, you can always go back for more, no matter where you’re at.