Boron nitride nanosheets (BNNs) are pursued for their high thermal conductivity yet electrical insulation and their extreme thermal and chemical stabilities for a variety of applications. For high-quality BNNs, a number of exfoliation approaches under different processing conditions were evaluated. Three more effective methods were identified, including method-A based on refluxing and vigorous sonication in isopropyl alcohol, method-B involving ball-milling in urea, and method-C with the aid of ammonia in hydrothermal processing. These methods produced BNNs of apparently different structural and morphological characteristics, with those from the first two being smaller pieces, more defective, and their polymeric nanocomposites exhibiting relatively lower thermal conductive performance. The method-C yielded BNNs of obviously better quality, as also made evident by the observed high thermal diffusivities of their corresponding polymeric nanocomposites. Correlations of exfoliation approaches and conditions with characteristics of the produced BNNs and consequences to their various applications are discussed.