The way I see it, you tip for extra complicated service.

At BK or Dunkin Donuts, you're getting a cup of coffee... no pumps, no "skinny", no half-caf, no foam options. If you're getting a drip coffee at Starbucks, I don't think that a tip is necessary.

I drink iced tea, unsweetened, and therefore do not tip, unless i really don't want to carry around those leftover coins.

If I come in with 4 drink orders for my office mates, and 3 different forms of payment ... I'm more likely to leave a tip.