So what lessons can retailers take from last year's holiday shopping season? Obviously, December wasn't a big disappointment for online retailers. Also, mobile shopping is an accelerating trend. Worldwide, e-commerce sales (B-to-C) are expected to top $1.7 trillion in 2015, with mobile commerce reaching $300 billion. And the impact of mobile goes well beyond actual sales. Many holiday shoppers used their smartphones and tablets to research products, plan shopping trips, locate deals and share their wish lists.