Key TakeawaysStarbucks' new CEO, Brian Niccol, officially started his tenure with the coffee giant Monday after announcing his departure from Chipotle last month.Niccol's appointment sent Starbucks' stock up roughly 25% in one day, with analysts largely optimistic about what Niccol can do for Starbucks.Starbucks' recent struggles in the U.S. and China, and pressure from activist investors to improve sales and stock price, will be key issues that Niccol will face as chief executive. Starbucks (SBUX) official ...