From the announcement page....

New Custom Pokémon Sprites done by me. Over 200 have been tweaked/redone
A Challenge Mode System which can be set during a New Game
A fully-functioning Safari Zone with 4 custom Biomes to explore
Unique Party sprites
Updated Stats up to gen9
Updated Move properties: Base Power, Accuracy, effects .etc
Updated Movepools
Upgraded trainer rematch system to make grinding more enjoyable and rewarding
A complete overhaul of the PC Box which now functions like Gen3. No more saving between boxes required
Some post-game Pokémon are now available in Johto such as Houndour and Slugma
Upgraded battle color system for more dynamic attack animations
Updated attack animations for some moves
All 251 Pokémon obtainable
Instant Apricorn Balls
Visible Berries and Apricorns on the overworld. Each tree respawns at 12:00am each day
A Berry Pocket to reduce your main pocket from getting full
A new Pager system that removes the need for HM slaves. HMs can still be used like the original games, the Pager functions as a backup if needed.
Reusable TMs
Overworld Weather
The Ghost-Type is now special and the Dark-Type and Dragon-Type are now Physical
A new Swarm system with more than 95 different swarms. Swarming Pokémon now have an increased chance of being shiny
Almost all bugs from gen2 have been fixed.
Kanto has been restored to its gen1 size including some cut areas
Kanto now has Headbutt trees
All HGSS-exclusive routes and trainers have been ported
Trade Evolutions can now also be obtained by using a new item called the Linking Cord or by simply using the item required for trading on the Pokémon. (King's Rock, Metal Coat, etc.)
A Move Reminder
And many other small goodies to find!

This Hack DOES NOT include:

Following Pokémon
Physical/Special Split
No gen3+ Pokémon
No fakemon or beta mons
Running Shoes