Guess I did not directly answer the question- really it is how they sound that makes them special but how they get there is the answer.
They use a lossy tuned cabinet resonance design that gives them a controlled richness.
The proprietary midrange driver material is important to the tonal accuracy.
The overall voicing is extremely important. Harbeth "voices" their speakers via the crossover components to give them a sound they want- through lots of iterative testing. What you hear is not the result of designing crossovers with simulation software and calling it a day. The crossovers are designed to make the speaker sound accurate and pleasing to the ear, not a computer program.
The crossover design also incorporates circuits to assist the drivers in sounding as good as they can by eliminating distortion, frequency peaks and making them easier to drive.
Lots of art expertise and back and forth testing goes into Harbeth speakers.
They walk that fine line between detail and forgiveness like no other speaker on the market.