The electronic structures of the ground state for several different superconducting materials, such as cuprates, conventional 3-dimensional superconductors, doped semiconductors and low-dimensional systems, are quite different and sometimes in contrast to what is supposed to make a superconductor. Properties like the Fermi-surface (FS) topology, density-of-states (DOS), stripes, electron-phonon coupling (λep) and spin fluctuations (λsf) are analyzed in order to find clues to what might be important for the mechanism of superconductivity. A high DOS at EF is important for standard estimates of λ′s, but it is suggested that superconductivity can survive a low DOS if the FS is simple enough. Superconducting fluctuations are plausible from coupling to long wave length modes in underdoped cuprates, where short coherence length is a probable obstacle for long-range superconductivity. Thermal disorder is recognized as a limiting factor for large TC independently of doping.