Fermi-surface fluctuations and lattice instabilities in the two-dimensional metallic kagome superconductor CsV_{3}Sb_{5} are elucidated via polarization-resolved Raman spectroscopy. The presence of a weak electronic continuum in high-quality samples marks the crossover into the charge-density-wave (CDW) ordered phase, while impurity-rich samples promote strong defect-induced electronic scattering processes that affect the coherence of the CDW phase. CDW-induced phonon anomalies appear below T_{CDW}, with emergent C2 symmetry for one of the CDW amplitude modes, alluding to nematicity. In conjunction with symmetry-breaking lattice distortions, a kinklike hardening of the A_{1g} phonon energy at T_{CDW} signifies a concerted interplay of electronic correlations and electron-phonon coupling in the exotic CDW order.
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