Achieving single-mode operation and highly directional (preferably unidirectional) in-plane light output from whispering-gallery (WG)-mode semiconductor microdisk resonators without seriously degrading the mode <formula formulatype="inline"><tex>$Q$</tex></formula>-factor challenges designers of low-threshold microlasers. To address this problem, basic design rules to tune the spectral and emission characteristics of microscale optical cavity structures with nanoscale features by tailoring their geometry are formulated and discussed in this paper. The validity and usefulness of these rules is demonstrated by reviewing a number of previously studied cavity shapes with global and local deformations. The rules provide leads to novel improved WG-mode cavity designs, two of which are presented: a cross-shaped photonic molecule with introduced asymmetry and a photonic-crystal-assisted microdisk resonator. Both these designs yield degenerate-mode splitting, as well as <formula formulatype="inline"><tex>$Q$</tex> </formula>-factor enhancement and directional light output of one of the split modes.