Terrain! That third (hopefully) painted force on the table that affects the other two. In this edition 40K has re-introduced some of the more bizarre types of terrain from earlier days that was largely ignored in the 3rd-5th years. Personally I like seeing some of this sort of stuff return, but prefer it to be more narratively created (and modeled) by the players in advance instead of the result of random rolls in the middle of the game.
We have the same basic types of terrain from before (p.90) - open ground, impassible, difficult, and dangerous. The main difference so far is that you can get armor or invulnerable saves against dangerous terrain, so things like terminators should feel a lot safer wading through it. Also remember that anyone with Move through Cover ignores dangerous terrain, like Beasts and Monstrous Creatures. Mindfield, shminefield.
There's also lethal terrain, which is like impassible but if you involuntarily wind up there you're just dead, no save, do not pass go. You probably won't see it on tables much, but when you do, look out for Pavane and Magna-grapples.
Area Terrain - always difficult, grants 5+ cover. Models in area terrain always get cover, even if not 25% obscured.
As far as I can see, there's no exception here for things like vehicles or monstrous creatures, so they should always get cover in area terrain too. (Oops, p.75 shows that vehicles still need 25%, even in area terrain.) Another change is that if you go to ground in area terrain, you get +2 to the cover save (so 3+) rather than +1. Lots of nooks and crannies, just like an English muffin.
Mysterious Terrain - similar to Fantasy, they have charts on p.102 and 103 for woods and rivers for random effects to reveal themselves in-game. You can of course agree with your opponent at the start to instead treat them as particular types (or even plain). No official charts for other types yet, but perhaps more will come in White Dwarf.
Buildings - (p.92) Intact buildings, conveniently enough like the ones you can purchase for your army as fortifications, but also covers other buildings you may scatter about the battlefield. These rules weren't used much in 5th, but I liked them and hope they'll be more prevalent in 6th. The rules here are expanded a good bit, providing for even more little cinematic bits like throwing grenades through fire slits and the like.
As before, buildings are treated like immobile transport vehicles, with an AV and transport capacity. There are also sometimes battlements on top, with rules for going up and jumping down if needed. Tip - don't jump down in you can help it - modified dangerous terrain tests that you can't take armor saves against. Ouch.
Unlike regular vehicles, buildings don't have Hull Points and have their own damage table. In addition, hits can harm the occupying unit - a glancing hit does a wound to a model inside with no cover, a penetrating does a wound with no cover plus rolls on the damage chart, which is modified for AP as vehicles (AP 2 = +1, AP 1 = +2). Damage results can reduce the structure's AV, do damage to the inhabitants as pieces fall down on them, or ultimately collapse or explode the structure. For example, on a Detonation! result (7) the unit takes 4d6 S6 AP - hits! If the survivors can't disembark, they're destroyed, so be very careful of letting the enemy surround a unit in a building!
Emplaced Weapons - (p.96) Not to be confused with Gun Emplacements (p.105) like the Quad-gun. Emplaced weapons are things like the heavy bolters on the side of a bastion. Can be manually fired, or auto fired at the nearest enemy at BS2 and fire last (at end of shooting phase, just before morale checks).
Fortifications and Dilapidation (p.96) - They say that if you stick fortifications on the table as neutral terrain - not purchased by either army, they are "dilapidated" - AV is 2 worse than normal, and emplaced weapons cannot be fired.
Ruins - (p.98) Difficult terrain, 4+ cover. They make a number of changes and distinctions.
- Ruins with bases decorated with rubble and such - count the base as area terrain. Those without bases (or presumably pristine bases) the ground floor is not treated as difficult or area terrain. Still difficult to move through the walls or go up to higher levels.
- Coherency is a bit looser from level to level to account for varying level heights.
- Only infantry, beasts, jetbikes, skimmers, jump and jet pack units can move to higher levels of ruin. So the MC can't, but a jump MC or FMC can.
- "Gravity - Nature's Downward Express" - as with battlements, can jump/fall down from high levels of ruins. As with battlements, bad idea.
- They note you should decide how abstract to make them, whether you need the ladders to move up and down, if you can move through walls and such or not.
Battlefield Debris - (p.104) Difficult terrain unless noted. A couple of pages of suggestions for various types of terrain and what to count it as, as well as the Gun Emplacement and Defence Lines rules.
Archaeotech Artefacts - (p.106) Another random terrain effect table that you can use as desired.
Unique Terrain - (p.107) A couple of examples of additional types and encouragement to create your own. Also a note on Terrain Datasheets that will start coming out in White Dwarf.
You may have heard a lot of rumors and people talking about how cover is so much weaker in 6th. In some respects that's true, with being able to Focus Fire and specifically target models out of cover, and smoke launchers being 5+. However cover is even easier to get for big targets (25% instead of 50%, auto cover in area terrain), better go to ground in places, being able to snap shot while gone to ground, plenty of things still giving 4+ cover...cover is still alive and well in 6th edition IMO.
Next up changes to the Force Org chart - allies and fortifications, and warlord traits!