For those players who haven’t been involved in the D&D playtest and are coming to the new rules with knowledge of earlier systems, it’s probably worth discussing how the system works.
As with 3E and 4E, the new combat system uses Cyclical Initiative. At the beginning of the combat, each combatant makes a Dexterity check and then everyone acts in that order, from highest check to lowest, for the rest of the combat. A Surprised character misses his first turn, but is ready to go in the second round.
Unlike most other versions of D&D, you can split your Move before and after your action – and even between individual attacks. This is glorious when you’re fighting fragile kobolds – you can run through several of them in a single round, despite them not all being adjacent to you, assuming you have enough attacks to do so. (Two weapon fighting and higher levels are the most likely source of extra attacks). It should be noted how different this is from 3E; in that edition, if you did anything but a 5 foot step, you could only make one attack. In the new edition, you can always make all your attacks, assuming you can reach your opponents!
A really big difference is how moving works when next to a creature. In 3E and 4E, if you moved from one square adjacent to a creature to another, you provoked an Opportunity Attack, where the creature got a swing at you. In the new game, you only provoke if you move away from the creature. It is quite legal to run in circles around an orc – you will not be attacked. When this is coupled with the ability of halflings to move through the spaces of creatures bigger than them (or anyone moving through creatures two sizes bigger), it allows for some very quick and unusual manoeuvres. If you have to get away, you can use a Disengage action instead of attacking to move without provoking attacks in the turn.
The net result of this is that, like AD&D, it is better to defend the wizard with a lot of highly armoured bodies rather than rely on a “sticky” fighter (as in 4E) to defend him.
In 3E, lots of things provoked attacks of opportunity, and quite a number of them made their way into 4E as well. These are mostly gone in the new edition. Only moving away from a creature will provoke one – and a combatant can only make one opportunity attack for each turn they take. (Your ability to perform a reaction resets when your turn begins). Spell-casting and ranged attacks no longer provoke opportunity attacks. However, if you’re adjacent to a hostile creature when you cast a ranged spell or make a ranged attack, you’ll have Disadvantage on your attack roll. Advantage and Disadvantage are two of the really good new mechanics in this edition, allowing exciting moments in combat, and enabling Bounded Accuracy. (Rolling two d20s and taking the best or worst roll changes the effectiveness of the action, but not the bounds of how good or bad it could be). Casting spells which require saving throws or, at least, no ranged attack rolls, may be done without fear of provoking an attack.
Critical Hits typically occur on the roll of a 20, although high-level fighters and some other abilities may increase that range. There are no rolls to confirm the critical, it just does extra damage. A few versions were used in the playtest, but the final version doubles how many damage dice you roll; unlike 3E, the damage modifier is not changed. There’s no maximisation of the damage dice, as there was in 4E.
The various Bull Rush, Disarm, Trip and Grappling special attacks are changed in the new edition. Disarm doesn’t exist (it’s a very problematic ability; when it was first introduced in AD&D, only a very few weapons could use it, and only against shorter weapons from the same list!) Bull Rush and Trip effects are in the game as Shove, and Grappling does not (yet) allow the full range of actions as in 3E. It allows you to stop another character from moving, and to drag them along with you, but little else. There is a Restrained condition, but no character can yet inflict that condition; we’ll have to wait until the Player’s Handbook
Two-Weapon Fighting is elegantly handled: you use a bonus action to attack with your off-hand weapon. This attack does not get the damage bonus due to Strength or Dexterity it would normally accrue. Certain classes and feats improve the off-hand attack’s effectiveness.
Characters unfortunate to lose all their hit points are now dying. There are no such thing as negative hit points in the game, although a blow that would have knocked you to negative hit points equal to your maximum hit point score will kill you outright. Instead, once you reach 0 hit points you start making Death saving throws: three failed saves, and you die. Three successful saves, and you stabilise. A natural 20 restores you to 1 hit point, and a natural 1 counts as two failed death saving throws. Being hit when dying also counts as a failed save, so you can see that it is possible to die very quickly once you’re reduced to 0 hit points!
One other thing worth noting is the Dodge action, which condemns all attackers targeting you to take disadvantage on their attacks. I’ve seen it used to great effect by a fighter with a high Armour Class blocking a doorway, whilst everyone else loosed missiles and spells at the monsters. An armour class of 18 is very hard for most low-level foes (and even those of moderate levels) to hit.
That said, creatures now give Cover once again from friendly attacks. If you’re trying to shoot someone who is standing behind the fighter, you’ll have a penalty – typically +2 to their AC (half cover) or +5 to their AC (three-quarter cover). Mostly, it will be +2 if the cover is given by a creature.
With relatively low hit points, combats should go past fairly quickly, especially at the Starter Set levels. At higher levels, they can take longer, but I’m yet to see one take as long as the longest combats in 3E or 4E. I hope you enjoy the game!