Business class just lost one of its biggest differentiators. When United Airlines unveiled its newest Polaris business-class cabin in March, it cemented the industry-wide shift to sliding-door suites. What was once a standout feature is now table stakes for US carriers as they compete for premium revenue. Delta Air Lines became the first of the Big 3 to adopt doors in business class in 2017; it one-upped itself on Monday with a new, even more high-end version, set to deploy on its Airbus A350-