Some of the most common features for which a user often selects Professional over Enhanced include:
-CLI Scripting
-Configuration as script load/save
-Link Aggregation
-Multiple Spanning Tree
-Subring for Mach1000 / RSR / PMICE / MACH4000
-SNMPv3 Data Encyrption
For a comprehensive listing of the features by firmware version, please reference the PDF linked below.
Links of Interest