In anticipation of the increasing demand of wireless traffic, WiFi standardization efforts have recently focused on two key technologies for capacity improvement: multi-user MIMO and wider bandwidth. However, users experience heterogeneous channel orthogonality characteristics across sub-carriers in the same channel bandwidth, which prevents ideal multi-user gain. Moreover, frequency selectivity increases as bandwidth scales and correspondingly severely deteriorates multi-user MIMO performance. In this work, we consider the frequency selectivity of current and emerging WiFi channel bandwidths to optimize multi-user MIMO by dividing the occupied channel bandwidth into equally-sized sub-channels according to the level of frequency selectivity. In our selectivity-aware multi-user MIMO design, SAMU, each sub-channel is allocated according to the largest bandwidth that can be considered frequency-flat, and an optimal subset of users is chosen to serve in each sub-channel according to spatial orthogonality, achieving a significant performance improvement for all users in the network. Additionally, we propose a selectivity-aware very high throughput (SA-VHT) mode, which is based on and an extension to the existing IEEE 802.11ac standard. Over emulated and real indoor channels, even with minimal mobility, SAMU achieves as much as 80 percent throughput improvement compared to existing multi-user MIMO schemes, which could serve as a lower bound as bandwidth scales.