"Betazoids are less prone to using sensory modalities the way you often send mental images by psi. Ferengi and similar species are immune to their - or Vulcan - psi, and may or may not block osanwë. There's not a known way for a non-psionic, non-immune person to block a Betazoid from their current thoughts, whether they're meant to be private or not; the defense for those people if they require one is to think about things they don't mind being read. I think those are the major differences."