I think the beans that they use are decent quality, but they seem to over-roast them, which leads to excessive bitterness, as well as killing some of the richer and more subtle flavors. Starbucks coffee does have MORE flavor than McDonalds/Tim Hortons/Dunkn' Donuts. Those other chains seem to just serve their coffee really weak. But the extra flavor you get from Starbucks is just a higher intensity of that burnt flavor. It makes you feel like you're getting a bigger caffeine hit (you're not), but it doesn't taste that great.

My personal theory, anyway.