Wireless LAN (WLAN, commonly called "Wi-Fi") has evolved from a single-antenna spread spectrum-based technology meant to primarily support data traffic to a MIMO-OFDM-based technology that supports ever-increasing video traffic. With the proliferation of smartphones and a widely-expected surge in the number of internet-connected devices in the near future, many future WLAN networks will be both dense and heterogeneous, i.e., networks with a large number of diverse WLAN devices that share limited spectrum and that operate in a relatively small geographical region. This prompted a fundamental reassessment of several underlying mechanisms of WLAN operation in the context of dense networks with a view to enhance them or add new ones as necessary. This multi-year effort has culminated in the IEEE 802.11ax standard (now at draft 1.0). We review some of the key elements in this new standard, including changes to physical layer processing, medium access control, spatial and frequency reuse, and energy-efficiency.