1. Damage dealing cantrips do not exist. Period. A player can make them first level spells if they want but their damage output does not go up another hit die or anything else because they are now higher than cantrip level. The former cantrip would have to be prepared and treated in every way as any other first level spell would be treated.
2. Death occurs at 0 HP. There is no three save rule. No stabilization or using a healing kit. You’re just fucking dead.
3. Short rests do not exist. Anything that would come back or be otherwise regained after a short rest comes back after a long rest.
4. Players heal at the rate of 5% of their maximum total full health hp per night of rest, rounded down. This is modified by adding their Con bonus as a percentage to that 5%. So a PC with an 18 Con adds 4% to their 5% per day for a total of 9% healing from an overnight rest. For a full day’s rest (24 hours of doing nothing in a totally safe environment) you add 2% more. So in the example above, a player resting for 24 hours straight, doing nothing but laying in bed, sleeping, eating, crapping, gets 11% of their max hp back in that 24 hour period. Have to be in a totally safe place to get that bonus though.
5. Magic Items with charges like a wand or staff have a set amount of charges that do not recharge with the new day. Typically they have a range as indicated in the 1e DMG. In order to be recharged, they need to go through the same process as when they were made, lessened a little bit in cost and time because the crafter has a formerly fully functioning item of that type.
6. D6 Initiative: group initiative on a d6 with the players choosing their order of action, or as per the book. Don’t dawdle!
7. Missile fire into melee combat: Firing missile weapons into melee is dangerous for both allies and enemies engaged in melee. Roll to-hit. If you miss, there is a 50% chance it might hit someone else. If a die roll determined that it did hit someone, the DM will roll to see who it might have hit, enemy or ally, by assigning each a number and rolling to determine which might have been hit, then roll another to-hit roll against that person’s AC to see if it did hit. If so, the player will roll damage. If players take the time to search the battlefield for arrows after combat, they can recover 50% of the ones they shot, and 75% for things like throwing axes or daggers. Magic items will have a higher survival rate.
8. Sneak attack, or backstab as it used to be called, is only possible if the rogue has surprise on his opponent. Surprise is determined by the DM, and will usually require a successful Stealth check. Yes, this means that sneak attacks can no longer be done in combat unless the enemy does not know you are there. Even then, it can usually only be done once, because as soon as you stick a sword in someone’s back they will know you’re there. Distraction, advantage or disadvantage, or flanking or having party members within 5 feet of the target are no longer factors in a successful sneak attack. Only surprise matters. You cannot hide in the middle of combat or when people are looking at you. Anything in the rules that allows for this no longer does. Being invisible doesn’t automatically give surprise.