The most distinctive feature of the popular Japanese mahjong variants (as opposed to other mahjong gambling games) is probably the own-discard furiten rule: players can't win off others' discards if any tile they previous discard during that hand would complete their hand. (This is distinct from and in addition to more common "sacred discard" restriction against winning off a discard after letting another winning tile pass during the same round.) To facilitate keeping track of this, in Japanese mahjong separate discard piles are kept for each player through the entire hand.
(The other distinct feature is the riichi yaku, which essentially lets a player (discretionarily) raise their stake and declare that the won't change their wait to increase the potential score of the hand on completion. This is pretty distortionary since it's by far the most common yaku at about twice as frequent as tanyao.
It is the reason "riichi mahjong" is a common name for the most popular family of Japanese mahjong variants. It's pretty distinctive. I rate it as less distinctive mainly because, when everyone at the table knows what they're doing, it mostly only affects gameplay around the edges, whereas the own-discard furiten rule has wide-reaching implications for "defensive" play.)
Aside from that, there's the usual mess typical of regional scoring variation.
I would add that one other important distinction, though I don't know how unique, is that you can't win off a hand which doesn't have any yaku. This partly explains the popularity of riichi, as it's okay to have riichi only.
