An essential feature of many modern teacher observation protocols is their "global" approach to measuring instruction. Global protocols provide a summary evaluation of multiple domains of instruction from observers' overall review of classroom processes. Although these protocols have demonstrated strengths, including their comprehensiveness and advanced state of development, in this analysis we argue that global protocols also have inherent limitations affecting both research use and applied school improvement efforts. Analyzing the Measures of Effective Teaching study data, we interrogate a set of five potential limitations of global protocols. We conclude by discussing fine-grained measures of instruction, including tools that rely on automated methods of observation, as an alternative with the potential to overcome many of the fundamental limitations of global protocols.