I'll just make a list.
1. The To Hit chart and the To Wound chart is re-jigged to resemble the shooting To Hit chart. Currently S3 can wound T4 on 5+, and T5 on a 6, and T6 on a 6 as well, and T7 not at all. I would prefer to see S3 wound T6 on a roll of 6 and re-roll of 2+. Likewise T7 would be a 6 and a 3+. T8 a 6 and a 4+, T9 a 6 and a 5+, and T10 a 6 and a 6. Conversely, S5 would wound T3 on 2+, and T2 on 2+ and re-roll of 6, and T1 on 2+ and re-roll of 5+.
2. Close combat is no longer WS vs WS, but WS vs I, with all attacks simultaneous. No more killing one side before it strikes back. This is organic to #1, so while most everything is hitting Orks on 3+ or 2+ or with re-rolls, they get their own mountain of attacks. Hitting stuff like Wyches and Daemon Princes, on the other hand, is something else...
3. Fearless is no longer complete immunity to Morale, Pinning, and Fear. It's just rolling 3D6 and choosing the lowest, much like how Move Through Cover lets a unit roll 3D6 instead of 2D6. Being Fearless does not prevent a unit from going to ground, or falling back. And They Shall Know No Fear would simply allow Space Marines that passed their regrouping test to act normally, instead of suffering the usual regrouping penalties.
4. Wounds ignored by Feel No Pain still count for the purposes of causing Pinning and Morale checks.
5. Multi-wound models will roll on the Vehicle Damage table when they suffer a wound and survive (do not suffer Instant Death). They will not explode if they roll a 7+ or 'Explosion' result. However, roll a dice to determine range as if they did, and morale checks to friendly models around it.
6. Armour saving throws and Armour Piercing would be re-worked to match the To Hit and To Wound tables, so that instead of either having a particular saving throw or not, they would have a different one depending on the relative AP and Sv. I would peg it at AP = Sv as need a 6 to save, and AP < Sv as needing a 6 and a 2+, AP < Sv-1 needing a 6 and a 3+, and so on:
AP6 ^ Sv6 = 6
AP5 ^ Sv6= 6 & 2+
AP4 ^ Sv6= 6 & 3+
AP3 ^ Sv6= 6 & 4+
AP2 ^ Sv6= 6 & 5+
AP1 ^ Sv6= 6 & 6
AP2 ^ Sv3 = 6 & 2+
7. Ignores Cover would only ignore a certain amount of cover, so like Feel No Pain it would have a number in brackets or a default 5+, meaning that like Shrouded and Stealth it would typically shift the roll by -2, or simply ignore Cover 5+. Yes, this is a gigantic nerfbat to Ignores Cover.
8. Destroyer Weaponry would multiply the number of wounds/penetrating/glancing caused by a D6.
9. Snap-shots will not all hit on a flat BS1. Instead they re-roll hits, and cancel out effects like Tesla weaponry that activates on a roll of 6. They would cancel out re-rolls for misses, like Twin-Linked weapons. Blast weapons and templates could be used for snap shots, since the blasts would just re-roll hits, and templates would inflict 1/2 hits, rounding up.
10. Instead of Overwatch, units would be able to fire assault weapons and pistols in combat as Snap Shots where models were not in base-contact. This would use the Assault To Hit chart instead of the Shooting To Hit chart, so it would be Snap Shots where BS vs I reckons the hits. Example, a Terminator armed with a Storm Bolter in an assault but not in combat would roll 2+ to hit Ork Boyz (BS4 vs I2), but re-roll any hits. Of course, the Terminator would be better off in base-contact using his Power Fist to hit Orks on 2+ and wounding on 2+ rather than 4+, and the Boyz getting a 6 and 2+ save versus a 6 and 5+ save. Getting close to the Boyz so that they can hit on 4+ with 2-3 attacks is pretty suicidal.
11. All units successfully engage in Assaults if the enemy is within 12". However, units still move 2D6", but don't necessarily have to close if the player does not want to, and get re-rolls/hammer of wrath, etc. Models can only use close combat weapons in base contact. They can engage multiple enemy units for the usual cost of a disordered charge.
12. As an option, units can return fire, or go to ground/jink, or fall back when attacked. They can also choose to do nothing. Units that fall back do so before the attacker checks range, so they can move out of range of the attacker (although they now need to regroup or continue falling back, and only fire snap-shots). Units that return fire must shoot at their attacker if they can, and otherwise cannot return fire. A unit can only return fire once in a player turn.
13. Units only get cover saves if they go to ground. While Skimmers can Jink, other vehicles can choose to go Hull Down and get cover saves if they're in or behind terrain. This prevents them from moving next turn, like infantry.
14. Terrain can be more or less dangerous, meaning that Dangerous Terrain may activate on more than a 1, and Move Through Cover does not ignore it, it simply re-rolls it like a goddamn Dozer Blade.
15. The pointy end of a template fired by a Torrent weapon must point at the model firing it.
16. Units that have already returned fire in a turn still have the option of going to ground if they continue to be fired upon, and if they have returned fire and gone-to-ground, they still have the option of falling back (or doing nothing). Units that have gone to ground or are falling back fire snap shots when returning fire.
17. Unwieldy weapons don't get a bonus attack for charging, unless mounted on a vehicle or monstrous creature.
18. Tank Shock is resolved in the assault phase, as an assault, using the Ramming rules to resolve wounds, and the tank itself to resolve hits like a template. Consolidate moves can be used to move out of the way! Calculate hits and wounds, then remove casualties, move survivors out of the way, and place the tank in its final position. Resolve attacks on the tank, including from the casualties. Resolve the assault as normal. Only tanks that moved at combat speed in the Movement phase and shot with Snap Shots in the Shooting phase can Tank Shock 2D6" in the Assault phase.
19. Daemonic Instability would mean that where the Daemons fail a morale or pinning check, they are removed from the board with no Sweeping Advance roll and put into ongoing reserve. If they don't return before the end of the game, then they count as destroyed.
20. Synapse allows Tyranid models within 12" of a Synapse unit to use that unit's Leadership when making leadership tests like Morale, Pinning, and Instinctive Behaviour. Instinctive Behaviour would also change. Feed would require moving towards and assaulting the closest enemy unit on a failed test. Lurk would mean the unit goes to ground. Hunt would mean shooting the closest enemy unit.
21. The Shadow in the Warp requires units to roll 3D6 for leadership-based tests and discard the lowest die. Cancels Fearless.
22. Psychic powers no longer have a dice pool. Each psychic power that a unit can cast (remember it can't cast the same power twice, even if member models have copies of the same power) is manifested by rolling 1D6. If the player rolls over the power's Warp Charge then the power is manifested. The opposing player can attempt to deny by rolling over that result on 1D6. A tie means the power is manifested. The usual bonuses to that roll apply (Adamantium Will, Psyker, Higher Level Psyker, etc). Where the power is not manifested, the psyker suffers the Perils of the Warp. Effects that make all doubles on power dice suffer Perils mean that any denial of a successfully manifested power also results in Perils of the Warp.
23. All Witchfires automatically hit, including ones with weapon profiles. The Culexus Assassin gets +D6 shots for each psychic unit within range. Spirit Leech adds a bonus to manifesting the Zoanthrope's following Warp Blast, but rolling 1 or 2 is still a Perils of the Warp even if the bonus takes it over the Warp Blast's Warp Charge of 2.
24. Instant Death is caused by weapons with a Strength 3 higher than the target's Toughness, so S7 will Instant Death T4. Instant Death itself is 1D6 unsaved wounds (still 1D3 wounds to Gargantuan Monstrous Creatures), and negates Feel No Pain.
25. Super-Heavy Vehicles and Gargantuan Monstrous Creatures roll on the Vehicle damage table if the attacker manages to roll 5+ after each penetrating hit or wound on 1D6. Feel No Pain rolls can ignore the wound, but not the damage effect.
26. Vehicles take morale and pinning tests upon suffering a penetrating hit, using either the Warlord's leadership, or the closest character of the same faction. Heavy vehicles fall back 1D6", regular vehicles 2D6", and Fast Vehicles 3D6". Failed pinning tests mean the vehicle goes Hull Down. Vehicles cannot be destroyed in Sweeping Advances. Units in combat with vehicles can make a 1D6 consolidation move, made before the vehicle may explode.