The main changes since the 2012 edition are listed here:
Red fives have been removed
Tanyao (All Simples) can be an open hand
Renho (Blessing of Man) is a mangan, not a yakuman
Dai suushii (Big Four Winds) is a yakuman, not a double yakuman
A hand with 13+ fan is scored as a sanbaiman, not a yakuman
The 3 second timing rule is removed
Swap-calling is not allowed
Temporary furiten ends when the player draws or claims a tile
Five counters implies two yaku: the rule is removed
Abortive draws are not allowed
Nagashi Mangan is not allowed
Uma is changed to 15,000/5,000/-5,000/-15,000
Chombo penalty in a tournament is 20,000 points deduction after uma
Penalty rules are slightly more lenient in some cases
Differences to World Championship Rules 2015
Multiple winners are allowed
4-30 is not rounded to mangan payment
Timing rules differ; EMA: pung takes predence over chow
Riichi bets at game end go to the winner
Motivation for the rule changes is to be in line with modern style riichi as it played in competion riichi in Japan and the World Riichi Championship 2015 rules have been the main inspiration.
The devaluation of Renhou and 13+ han hand is for the same reason as previous removal of accumulative and double yakuman: hands that are very valuable skew tournament results from skill towards luck, and should be avoided in tournament play.
The chombo payment of 20,000 *after* uma is quite close to paying a reverse mangan to the opponents immediately, maybe event sligthly cheaper. The reverse mangan would often equivalate dropping one or even two positions at the table, that's at least 10,000 points on top of the 8,000 or 12,000 paid for the mangan. The reverse mangan is paid to the opponents who benefit from the mistake, and the dealer gets a larger benefit. This is now avoided.
The abortive draws and Nagashi Mangan were the most difficult rules to consider. People have strong feeling both for and against Nagashi Mangan in particular. It seemed the split for/against were very close to 50/50, and in the end we just had to make a decision. These decisions cannot be strictly objective and well-argued, because in the end it comes down to preferences. In the stalemate circumstances I suggested to use the WRC rules as a guideline.
The rules are available in a preliminary document while further layout and graphics is made in collaboration with Sheila Hansen who also designed the front page.