Part 1. Removing Specializations
Removing true definitive specs such as Fire Mages, or Marksmanship Hunter. Classes just have an overarching theme and will have a set, baseline rotation that will be further customized based on how your use your new talent/trinket point system. which will be explained later. This allows classes to be less ridged and specs no longer need balancing limitations within classes.

Some examples. Lets say you always wanted Gladiator stance warrior back from WoD for warriors. Now you can DPS as Sword and Board. lets say you like the "fury" style tanking like in classic. You can now tank Dual Wielding.

Lets say you miss WOTLK DKs. You can choose to customize your character with primarily blood, frost, or unholy abilities. All usuable for both tanking and dps. You can use a 2hander again with a frost focus. You can dual wield focusing on blood or unholy abilities.

Monks can true fistweaver heal again . Priest can be a holy spellcaster DPS. Hunters can emmulate old ranged survival. You can be a triad mage focusing on all 3 elements. So on and so forth. How your character plays is completely up to you and how you design it.

Part 2. Talents and Trinkets
Talents are largely redesigned. You now have a total of 15 talent points 5 for each category. Talents are divided into 3 categories.
Offense, which will primarily be more damage rotation spells. These will always have primary active abilities
Defensive, which will primarily be sustain options, or crowd control. These will always have primary active abilities
Utility, which will primarily be passive, but may alter abilities, such as baseline passive ones to active abilities.

There are 15 options per category for a total of 45 per class. You have 5 talent points to put into each category, and each Offense/Defense Talent have additional 4 sub augmentations (think D3 runes) that further enhancement the ability. Augmentations can be freely choosen and changed as long as you have that ability learned.

An example Offense Talent for a Warlock
Summon Felguard: Summoning a demon will also summon a powerful Felguard who cleaves his current target. The felguard follows the same commands as your regular demon. Additionally granting the Fel strike ability.

Soul Strike: Command your Felguard and Demon Pet to charge at the target and attack, dealing X damage. 2 charges. 5 second cooldown.

An example Augmentations
Houndmaster: While a Felhunter is active, the Felguard emits an aura that increases the damage by 30% and movement speed by 200% of your Felhunters and Dreadstalkers. In addition, Dreadstalkers basic attacks apply Fel Weakness, increasing all Chaos damage you deal to the target by 75% for 3 seconds.

Devour Shadows: While a Voidwalker is active, every soul shard spent will increase the voidwalkers damage and size by 10% for 30 seconds. This effect can stack but does not refresh. In addition, any non player, including your own minions that dies nearby will add 5 stacks, and any player that dies will add 15 stacks. Maximum of 50 stacks.

An example Defense Talent for a Shaman
Earthen Wall: Summon a large wall of stone in the target area with x health for 10 seconds, knocking enemies back slightly. The Earthen Wall prevents line of sight and and will block movement until it is destroyed or expires. Can be reactivated to cancel. 2 minute cooldown.

An example of Augmentations
Frostshards: The Earthen Wall is now made of ice. Its size and health is increased by 25% and when broken will root nearby enemies for 4 seconds.

Windwall: The Earthen Wall is now a vicious Wind Tunnel in front of the casting shaman and the duration is increased to 15 seconds. This windtunnel  will continuously push back enemies. Enemy players are less affected. (Think Falstad's level 20 Mighty gust in HoTs). Cooldown increased to 3 minutess.

An example Utility Talent for a Hunter
Aspect of the Pack: Your Aspect of the Cheetah now has a 20 second cooldown and lasts permanently, but the movement speed bonus is reduced to 40% and is removed upon taking damage beyond the first 1.5 seconds of activation. Applies Aspect of the Pack to nearby party members, granting them the effect aswell.

An example of Augmentation
Thrill of the Hunt: Whenever someone in your party or your pet is snared/rooted, you gain 40% movement speed. This effect will last permanently or until taking damage beyond the first 1.5 seconds of activation.  This has a 6 second cooldown.

Part 3: Glyphs

While not necessarily tied to combat, the glyph system is gettiing majorily overhauled. It is now a  cosmetic system for your spells which are rewarded from endgame content seasons, which will be PvP seasons both RBG and Arena, M+ seasons Raid tiers while current, and from other activities such as a new Pet battle ladder.

Glyphs are now something that has to be earned and will not longer be obtainable once a new seasons (or major patch cycle) happens. They will be tied to be thematic to whatever that season was about and give a flair to someone of your baseline abilities.

For example

BFA S3 Raid Reward for Mages: All your frost spells, abilities, and talents now have a more watery appearance
BFA S4 Arena Reward for Shamans:  Your totems are replaced with special Void totems