So I thought I'd nail down the sheet for a quick guide to the differences between the heroic and normal modes.   By default, everything hits harder and has more hp, listing mechanical changes.   See strat pages for more details.

Marrowgar:

Bone Spikes during Bone Storms - This causes problems in that people can die really fast.  Spread out in hopes that it lowers chances of him spinning on top of a spike.   So don't clump up for spikes, this means you melee.  Only kill what's conveniently in your area, focus on survival.    Does mean healers have to keep Spiked people up as they'll remain in their longer.

Coldflame is aimed more - Slightly, it lasts longer moves faster and will be fired towards you.  Straight line still, so move away faster

Deathwhisper:

Mind Controls - Starts doing it to 1 person in 10 mans, bumps up to 3 people in 25 man.  So more CC.

Faster Add Waves - Kill kill.

Dark Martyrdom is lethal - This goes under the category of everything hits harder, but special note that this really really hurts.   So melee run away.

Phase 2 is not Tauntable - Gotta do tank switches like Gurtogg, 2 tanks both get highest threat.   It is possible to one tank it with enough MDs/Tricks going to him.

Phase 2 adds still occur - But less numbers.  In 10 man, the extra add at the back you see in 25 man, that will start spawning.   In 25 man, it'll only spawn one side at a time, so like how 10 man is.

Vile Spirits now explode in 20 yard radius - Before it was a targetted attack on someone, now they explode.  So if they don't run, you all get burned.

Gunship:

Rockets knock you back 7 yards - That's all.

Saurfang:

BP Generation up - This kinda goes with the hits harder theme

Adds will do a slowing aoe - Some seconds after they spawn they will emote "Blood Beasts smell the scent of blood" in which they do like a aoe around them that puts a movement slowing debuff, and increases their own damage by 300%.   Just means kiting has to be perfect or things die.

Note - In 10 man, we 2 heal this to get it all down faster.   But hybrid it to switch to 3 healing at the end.

Rotface:

Vile Gas - Putricide joins the fight and throws Vile Gas on the ranged.  Keep 3 minimum at ranged now, 7 in 25 man.  The Vile Gas is similar but not Festergut's version.   It does damage people around you, but it only roots you for a couple seconds, not the disorient so can still cast and all.

Festergut:

Malleable Goo - Putricide joins the fight and chucks these at the raid.  Just like the one he throws himself.   The raid should be spread out as much as possible so Putricide has more targets to aim at.    Try not to get hit.   It is healable, but with the cast/attack slowing effect, it's gonna make it tougher on you.

Putricide:

Unbound Plague - Think like the Frost Giant weekly except longer.   There's a plague for every second it's on you it'll put a stacking debuff which increase it's damage done to you.  It also jumps to the nearest player, prolly within 8 yards.   The goal is to pass it around the raid, about 5 players can do it, 1 -> 2 -> 3 -> 4 -> 5 -> 1, or something.   So spread out, take a few sec of it, then run it to the next guy, spread out again.

Transition Phase spawns both adds - Instead of a Tear Gas stun, he'll spawn both Gas Cloud and Volatile Ooze at the same time.   He himself willl still go through his roleplay drinking potions for a bit so you have time to deal with these.   Suffice to say, kite Cloud, grill Ooze.

Gas and Ooze Variables - Only in 25 man, but they make random members of your raid only able to attack Gas or only able to attack Ooze.

Blood Princes:

Shadow Prison - Every time you move, you get hit with shadow damage, and it stacks every second you move.   Lasts 10 seconds.    Similar to Hodir but opposite, moving hurts you.   But it's not a dot, it's damage per movement.

Damage Note - The fight is the same otherwise, but due to the damage increases, basically means you'd need another near perfect execution.  Kinetic Bombs are lethal, Empowered Fire Orbs are lethal, etc, etc.

Blood Queen:

Shadow Aura increases as more Vamps are out - That's about it.  Well again another tight execution due to higher damage from everything.   Also might want to consider staggering bites in 10 man cause you will run into the problem of the final bites occuring right at the 2nd fear.

Valithria:

Shadow Aura - There's an aura now that hurts her and the raid, so she'll start dropping hp at first.   Emphasis on stacking fast and never losing them.    (and adds spawn faster)

Sindragosa:

Instability backlash is a 20 yard explosion - So if they fail in the group, the group will know it.  Basic goal is to run out to do your thing.   Alternatively, casters just cast 1 spell, heal through that, cast 1 spell again, repeat.

Lich King:

Shadow Traps in Phase 1 - Black swirly things I think, you touch him, you're gone (huge knockbacks)

Valks stay up in Phase 2 - You get them down to 50% or so, and they drop the guy, but they fly up into the air and start Siphoning Life from the raid.   Ranged can still kill them.

Everyone gets to see Shadowmourne in Phase 3 - Yes, everyone gets sucked in, the outside Vile Spirits will be stunned in the meantime like Yogg tentacles.    Inside, there are other spirits in the air, they drop bombs, and they hurt.   Ranged kill those.