I understand there's already a lot of translations floating out there, and the official O&A thread is actually quite complete as well. But for the sake of a clean summary, here's what we know:

(Please understand that there are overlaps of information here. Feel free to add all this to the existing compilation thread instead)

2.28

Due to delays, the release date of patch 2.28 has been pushed back to June 4th or 5th.
"Exploration Log": A series of locations that players will need to reach through climbing/jumping. Locations will be provided through hints and riddles that players will need to decipher.
New objectives will be added to the existing Challenge Log.
Repairing with a crafter will add 100% durability to the existing equipment durability, up to a maximum of 199%.
Equipment preview will become available in NPC shops, Market Boards, and crafting menus. May also be implemented with treasure chest gears and quest rewards in the future.
As mentioned previously, PvP skills will become unlocked from the start.
It will now be possible to queue for Expert/High Level roulettes with a pre-made party.
Titles can now be displayed with player names.
Echo buff will now also apply to pets.
You can now see the progress of FATEs on the area map.
You will be able to teleport to aetherytes by selecting them on the map.
Added option to automatically skip previously viewed cutscenes.
Leviathan and King Mog Extreme fights can now be queued solo in Duty Finder.
It will now be possible to give out commendations in pre-made groups (only to party members who queued solo).
Phantasia potions will become available for purchase. Initially only in quantities of 1, 3, or 5 (can only purchase each option once, so maximum of 9 can be purchased). Option to purchase as many as you need will be made available in the future. Pricing is set for around 1000 yen each.

Zodiac Weapon

There is no requirement for starting your Novus weapon quest other than being in possession of the Animus weapon.
Some level of customization will become available starting with Novus Weapons.
Powering up your Animus Weapon will require "Sphere Scrolls", which itself requires reagents called "Alexandrites". Figuring out ways to obtain alexandrites was the reason why Novus was delayed to 2.28. More ways to obtain alexandrites will also be introduced in 2.3.
There is no plan to ease the mechanics for obtaining Atma/Animus weapons. However, the amount of Mythology tomes obtained from contents will be increased, so it will become easier to get your books.

2.3

Introduction of 1 new dungeon (Hullbreaker Isle), 2 hard mode dungeons (Tam-Tara and Stone Vigil), and 1 new Primal (Ramuh).

Ramuh

Like Leviathan, Ramuh will have Hard (story) and Extreme modes.
Completion of Sylph beastman quest chain not required but will add context to the story.
ilvl 100 weapon will be a rare drop in Ramuh Extreme. Regular drop will be accessories.
Mount will also be a drop (probably lightning unicorn). Collecting all primal mounts "may get you something good in the future".
No confirmation on whether or not there will be "falling" (permanent removal) mechanics in the Ramuh fight (the insinuation is that there will be. Probably.).

Hullbreaker Isle

Hullbreaker Isle will involve a character from the main story.
Will decide on the final ilvl requirement closer to the release date.
A familiar Kraken monster will appear as the boss. Will be a different sort of fight from your usual boss fights.
ilvl of gear drops here will be determined closer to the release date.
New minions? Maybe, maybe not.

Item Salvage

Simply put, the crafter ability to break down items to obtain crafting materials.
Items can only be salvaged by their corresponding crafting jobs, similar to materia melding.
Salvage skill becomes available at lv30.
Salvage skills can be leveled up through usage (higher level = higher ilvl items you can salvage). But the level cap is shared by all 8 crafting jobs so you will need to choose between specializing in 1 job or distributing between jobs.
Once at skill level cap, further increasing the salvage skill of one job will cause another's skill level to drop.
Not all, but many items can be salvaged, including gears, furniture, and even certain fishes.
For the most part, salvaging an item will give you a material that was used to make the item. You will not be able to salvage 100% of the crafting materials used to make the item (obviously). Certain rare items can only be obtained through salvaging.

Mob Hunt (aka. Marks)

"Risky mobs" that can be defeated for points.
Three ranks available: S, A, and B.
You can hunt these marks any time you want to earn points.
One monster (Rank B was used as an example but possibly A and S rank as well) will be designated by your Grand Company weekly. Defeating the designated mark will earn you bonus points on top of what you normally get.
Another designated mark will be assigned daily to each player separately (so not everyone will be hunting the same thing) for bonus points (similar to how daily roulette provide bonus tomes).
Points can be used to obtain Myth items, as well as alexandrites used for Novus weapons. Mostly though, the rewards will be unique and cannot be obtained elsewhere.
Difficulty ranges from just barely solo-able with full ilvl90 gear to needing a chocobo companion to things that will require a group of people to beat. So if you run into a hard one you may need to shout for help.

Black Mage Buff

Single target damage will be buffed.
To maintain balance, other adjustments will be made (does not specify whether AoE will be weakened or maybe even involve adjusting other jobs). But overall Black Mage DPS will increase (when it comes to those single-target encounters).

Chocobo

Skill point reset will become possible in 2.3.

Crystal Tower

ilvl 110 upgrade items (oil/sand) will be dropped here.

Difficulty of Contents

Despite complaints of many fights feeling "scripted" and lack random elements, it is a deliberate attempt to prevent difficult fights from becoming overly luck-based, which tends to add to player stress.
Considering the growing number of people who have already cleared the Second Coil, there is now something in the plans that is designed to be impossibly difficult for these players to challenge. The reward will only be a title, but it is something that only the best of the best would be able to obtain.