If you're over 50, you've probably had this experience: You're standing at a checkout counter, ready to pay, and the twentysomething behind the register is talking on her cell phone. So you wait, and wait, and wait, and when the clerk finally finishes her conversation, she offers not an apology, but a grimace that suggests you've interrupted. Sound familiar? It has a name: the Service Gap. That's not a hip clothing store for soldiers. Or a new motto for the London subway system. It's business-speak to describe a phenomenon fueling plenty of holiday-shopping frustration: the difference in how baby boomers...