I like Dunkin Donuts and many other coffee shops. The The only thing I don't like from them is poor customer service.
I don't want outstanding service. Normal will do.
For example, if you're unsure what the customer ordered read back what you show on their receipt and ask if that was everything. Make sure it's right.
Seems simple enough. Customers aren't ordering a side of attitude, just some coffee and maybe a donut. If employees can't handle mistakes and decide to take it out on customers they should have a different role. Customer service isn't for everyone. It should be a requisite for those on the front lines.