Defect modes and related transmission peaks arising in photonic crystals (PC) due to deterioration of the periodicity have extensively been studied since about ten years in connection to various applications related to optical and microwave engineering. In this paper, we study plane wave scattering by thin slabs of dielectric PC with air and metallic defects. The aim is to consider some effects exerted by single and multiple line defects on transmittance within and beyond structural gaps and on the corresponding field patterns. Calculations have been performed using the recently developed fast coupled-integral-equations technique, for both frequency-independent and frequency-dependent descriptions of the permittivity of regular rods and defects. The emphasis in the numerical study is on the effect of the angle of incidence on the transmission behaviour related to the presence of defects within and beyond structural gaps, on variation of the field pattern in the vicinity of defect-mode-originated transmission peaks, on poorly known regimes of the field localization arising due to defects, and on superposition of structural and polariton, and structural and plasmonic gaps. Several numerical examples are presented, which illustrate some interesting manifestations of the defect-related regimes.