Some big things you definitely need to know, and a few tweaks that are good to know. This isn’t an exhaustive guide, just a heads-up. So don’t argue with the umpire unless you’ve read the actual rules here. Or you can download the very handy FIH Rules app available from Google Play or Apple AppStore. England Hockey's summary of these rule changes is here.

(Better still, just don’t argue with the umpire and get on with the game instead.)

BIG CHANGES
NO MORE KICKING BACKS: Either 11 outfielders, or 10 outfielders and a proper goalie. If you choose the former, no-one can kick the ball, not even on a penalty corner. If there’s a flick, the chosen defender has to wear a helmet.
See changes to rules 2.2 (Mandatory Experiment); 13.3.g (team’s responsibilities when defending a Penalty Corner); and 13.7.e (protective equipment for the outfield player defending a penalty stroke).

COMPLETELY NEW RULE AT SHORT CORNERS: If any attacker breaks early at a short corner, the injector (not the early-breaking attacker) has to go to the halfway for the re-take.
See changes to rule 13.6.e (where an attacker enters the circle at a Penalty Corner before permitted).

SIXTEENS: Or fifteens. You know what we mean. A free hit awarded to the defence within the defensive circle can now be taken anywhere in their circle, OR taken up to 15 metres from the back-line in line with the location of the offence. Encourages quicker restarts, to the advantage of the non-offender.
See changes to rule 13.1 (free hit awarded to the defence).

THE NEVER-ENDING CHANGES TO THE 5m RULE NEAR THE CIRCLE: In a nutshell, the exception for defenders shadowing attackers when a free hit is awarded (to attackers) near the D has been amended so it only applies if the attacker immediately self-passes. Rule of thumb: everyone has to keep 5m away.
See changes to rule 13.2 (additional guidance concerning the re-start after awarding a Free Hit to the attacking team within 5 metres of the circle they are attacking).

SOME OTHER TWEAKS
THIRD PARTY OBSTRUCTION: Minor clarification that, at a penalty corner, attackers can’t run in front of defenders (or the goalie) and so block them running out (changes to rule 9.12).

DISCARDED EQUIPMENT IN THE CIRCLE: Clarification that if the balls hits anything lying in the circle and a goal is prevented, a flick may (probably will) be awarded (changes to rule 12.4).

FEWER BULLIES: Injuries during a penalty corner should result in a re-take of the penalty corner – not a bully (changes to rule 13.5).

ENDING SHORT CORNERS: If the full or half time whistle goes during a penalty corner, the option that the penalty corner is completed when the ball travels outside the circle for the second time no longer exists.