Shared structure
Both styles use four sets and a pair, chows from the left, pungs and kongs from anyone, and special hands such as thirteen orphans. A Hong Kong player can follow a riichi game almost immediately.
The yaku requirement
Riichi requires at least one yaku, while Hong Kong requires a faan total (sometimes zero). In riichi an open hand with no yaku cannot win, which shapes calling decisions profoundly.
Defense and information
Riichi's ordered discards, furiten rule and riichi declaration give defense a central role. Hong Kong play is more offence-oriented, with defense guided by reading discards in a shared pool.
Bonuses
Hong Kong uses flowers and seasons; riichi uses dora, red fives and ura-dora. Both add excitement, in very different ways.
