What actually changed in 11th edition.

In 10th edition your whole army used a single detachment, picked once at the start of list-building. In 11th, a list can include more than one detachment at the same time, mixed together.

Not in the core rules — army building and missions are published separately.

Each detachment costs Detachment Points (DP) — roughly 1 DP for something narrow that only affects a few units, up to 3 DP for something that boosts your whole army. How many DP you have to spend depends on your list's overall size. Some detachments are also tagged "Unique," meaning you can't take two of that same tag in one list.

Not in the core rules — army building and missions are published separately.

There's no longer one big fixed list of stratagems per army. Instead, the detachments you chose determine which stratagems you have access to — a typical list ends up with somewhere around six to nine.

Not in the core rules — 15.01 only says extra stratagems come from "codexes and other publications", never mentioning detachments.

A unit can only be affected by one stratagem in a given phase, and you can't use the exact same stratagem twice in that phase. Stacking multiple stratagems onto one unit in a single phase, which was possible in 10th, no longer works.

15.01Using StratagemsCore Rules p.54 →

Cover no longer improves your armour save. Instead it makes you harder to hit in the first place — the attacker usually has to worsen their hit roll by 1 against a target with the benefit of cover.

13.08Benefit of CoverCore Rules p.50 →

Both cover and the Stealth ability apply the same -1 to hit — having both at once doesn't give you -2, since it's a single on/off benefit rather than a stacking penalty.

No single rule says this. It follows from reading the two below together: Stealth doesn't grant its own penalty, it grants the benefit of cover, and the benefit of cover is one fixed -1 however many times you qualify for it.

24.33Stealth — grants the benefit of coverCore Rules p.85 → 13.08Benefit of Cover — where the -1 livesCore Rules p.50 →

Terrain marked as Obscuring blocks line of sight completely through its footprint — if the line between two models crosses an Obscuring terrain area, they can't see each other, unless both are inside that same terrain area.

13.10ObscuringCore Rules p.50 →

Terrain with the Solid keyword blocks line of sight through small gaps near ground level — a doorway or window in a ruin no longer gives you a sightline through the building.

13.11SolidCore Rules p.50 →

Infantry, Beasts, and Swarms sitting inside a dense terrain feature who haven't fired this turn or last turn can't be targeted by anything more than 15" away — they're effectively hidden until they shoot or an enemy gets close.

13.09HiddenCore Rules p.50 →

A unit with the Towering keyword within 12" of its target, or any unit standing on terrain 3" or taller, gets +1 to hit against targets at ground level.

22.05Plunging FireCore Rules p.73 →

10th edition scored objectives using small circular markers placed on the table. 11th removes them entirely — the whole footprint of a terrain feature is the objective, and control is worked out from which models are actually standing in or on that terrain.

14.01Terrain ObjectivesCore Rules p.52 →

Primary objectives are drawn from a mission deck and can be influenced by what kind of army you brought, so the two players in a game don't necessarily have identical primary objectives anymore.

Not in the core rules — army building and missions are published separately.

A unit tests for Battle-shock if it's at or below half its starting models, not strictly below half. A two-model unit that's lost one model can now be forced to test.

08.03Battle-shockCore Rules p.30 →

A unit that's already Battle-shocked has to test again to recover — it doesn't automatically shake it off at the start of the next turn. A badly-shocked unit can stay useless for several turns in a row.

08.03Battle-shockCore Rules p.30 →

When a unit is attacked, its controller now declares the order damage will be allocated across the unit before making any save rolls. Characters still take damage last.

05.03Save RollsCore Rules p.19 →

Monsters and vehicles only get the benefit of cover if terrain is actually blocking line of sight to them, not just because they're standing inside a ruin's footprint — infantry-scale units get an easier route to cover that vehicles don't.

13.08Benefit of CoverCore Rules p.50 →

The Heavy keyword's +1 to hit applies in your Shooting phase only, and only if all three of these are true of the firing unit: it isn't engaged, it wasn't set up on the battlefield this turn, and no model in it moved more than 3" this turn.

So a small shuffle to clear line of sight is fine, but a real move, arriving from reserves, or being tied up in combat each cost you the bonus. It applies to any unit carrying Heavy weapons, not just vehicles.

24.16[Heavy]Core Rules p.82 →

Weapons with the Psychic keyword can ignore modifiers to their Ballistic or Weapon Skill and to their hit roll — cutting through things like cover's -1 to hit. This doesn't extend to wound-roll modifiers.

24.29[Psychic]Core Rules p.84 →