Additive manufacturing (AM) has garnered attention as a promising production method for Ni-based superalloy 718 (Alloy 718) components with complex geometries. Despite the considerable potential of AM technology, challenges continue to hinder its widespread adoption. Two prominent issues are the small defects and complex microstructural characteristics induced by the AM process, both of which complicate the evaluation of fatigue strength. While many studies have focused on the impact of defects on the fatigue strength of AM alloys, systematic investigations linking microstructural features to fatigue behavior, particularly near the small fatigue-crack threshold regime, remain limited. In this study, fully-reversed push-pull and torsional fatigue tests were conducted on AM Alloy 718 specimens. To isolate the effects of defects and microstructural characteristics, the fatigue limit was evaluated based on the shear-mode fatigue threshold, with quantitative consideration of the effect of defects. The results revealed that AM material exhibited inferior fatigue resistance compared to wrought (WR) material, regardless of defect size. Against this background, the microstructural factors contributing to the inferior fatigue crack-growth resistance of the AM alloy were comprehensively discussed. First, the effects of precipitates (i.e., hardening precipitates, δ phases, and Laves phases), grain size, and residual stress were investigated. Additional fully-reversed push-pull fatigue tests were performed on another AM material with a different microstructure. The two AM materials differed in build rate (BR) and the presence of hot isostatic pressing (HIP). The original material was produced at a higher BR without HIP, while the new one was built at a lower BR with HIP. The lower BR and HIP process resulted in a fine grained, homogenized microstructure with likely negligible residual stresses in the new material. Nevertheless, the test results indicated that the shear-mode fatigue threshold of the new AM material was nearly equivalent to that of the original, suggesting that the influence of precipitates, grain size, and residual stress was minimal. Second, the influence of crystallographic texture and annealing twin boundaries was examined in terms of grain misorientation. Specifically, the distribution of the twist angle between two adjacent crack planes at grain boundaries was analyzed along the crack path of both AM and WR alloys using electron backscatter diffraction and stereographic projection. It was concluded that the twist angle showed a strong correlation with the difference in shear-mode thresholds between these materials, identifying it as a possible dominant factor affecting small fatigue crack-growth resistance.