Molten Core changes in WoW Seasons of Mastery!

Lucifron
Boss now has four Flamewaker Protectors, instead of two

Magmadar
After 20-30 seconds after starting the boss fight, he will spawn two Core Hounds near the Magmadar. The new boss mechanic will work like "dogs" in Molten Core, which should be killed at the same time.
Core Hounds will continue spawns every minute.
Lava Bomb - will casted more often and leave more void zones.

Gehennas
Boss now has four Flamewalkers instead of two.
Rain of Fire will casted more often.

Garr
During the boss fight Magmakin will spawn on top of Garr and charge to random raid member, and then explode.

Magmakin was in files of the old Classic client, but Blizzard decided do not to add this mechanic in the game. And now we can see this in WoW Seasons of Mastery!

Baron Geddon
Living Bomb now applies on three targets, instead of one!
At the end of Living Bomb and Inferno, a fire patch will be left on the ground that deals damage to anyone standing inside of it.

Shazzrah
After casting Blink, the boss will leave Image of Shazzrah, which will cast Arcane Explosion for the rest fight.

Golemagg the Incinerator
Golemagg's Core Rager pets are now buffed by Golemagg's Trust. This buff increases their physical damage and attack speed.
Golemagg now casts Earthquake during the all-fight, but before he used it below 10% HP.

Sulfuron Harbinger
Sulfuron and his four Flamewaker Priests now have a shared health pool.
Inspire which increased Physical damage and Attack speed, is now a bit buffed. And all of these priests can buff each other with Inspire.

Majordomo Executus
Flamewaker Elite now can cast Fireball Volley.
Flamewaker Priests become immune to crowd control after two deaths, instead of four.

Ragnaros
Ragnaros will force submerge phase when total health remaining reaches 50%. So now you can't kill Ragnaros without submerging phase.
After Ragnaros respawns from the submerge phase and for the remainder of the fight, Son of Flame can spawn in random locations from the Lava.
Also, Blizzard adds a 60-minute timer for despawn Ragnaros. So bye-bye 3-hours Molten Core runs.

In conclusion, I would like to say it is an amazing Molten Core update. As we see Blizzard did it really harder and restored the old mechanics looks really cool.
I think it's just the beginning of building a True Classic experience!
See you on beta!