The improvements consist of the Black Pits (stand alone adventure), three new NPCs (+ new adventures/areas associated with them), high resolution support, newly coded UI, BG1:EE ported to BG2 engine with all its kits and improvements.) Here is a list from their forums:
ENGINE ENHANCEMENTS
Based on the Throne of Bhaal engine.
BG:EE is based on the latest Throne of Bhaal code, which means it includes all features from Baldur's Gate: Shadows of Amn and Throne of Bhaal, as well as all bug fixes from those titles and their relative patches.
Rebuilt Threading Model.
The threading model has been cleaned up and fixed, thus eliminating the no-ops that characterized the original and improving performance. Several "Windows-95isms" were removed to make the engine run smoother and increase new-OSs-friendlyness.
Rebuilt Renderer.
The renderer was also cleaned up, and ported entirely to OpenGL for better performance. The graphics are now 100% hardware accelerated.
Fixed up multiplayer.
Baldur's Gate's buggy multiplayer support has been fixed up and improved, eliminating bugs and introducing cross-platform compatibility. It will be possible to play a multiplayer BG:EE game connecting a Windows based PC, an Apple Mac PC, an iPad and an Android tablet simultaneously.
ToBEx engine fixes.
The Overhaul team is working with Ascension64, author of the mods Throne of Bhaal Extender (ToBEx) and Baldur's Gate Trilogy (BGT) to improve the engine and bring over all the major fixes from those packages.
Extensive bug fixing.
BG:EE will feature a huge amount of bug fixes as suggested by the community, found by devs and beta testers, and found by modders throughout the years. Several bugs that currently require Baldurdash, Dudlyville, Tutufix, BG Fixpack or BG2 Fixpack - as well as a plethora of hardcoded bugs that require access to the source code to be fixed - will not exist in BG:EE.
VISUAL ENHANCEMENTS
Native support for high resolutions.
BG:EE natively supports high resolutions in 4:3, 16:9 and 16:10 format up to 2880x1800. The source area art will be filtered via hardware to scale it as required and enable the zoom in/out feature.
Dynamic zoom in/out feature.
It will be possible to zoom in and out of the battlefield dynamically during gameplay by scrolling the mouse wheel up and down, within generous limits.
New, high resolution cinematics.
The original, low resolution 3D cinematics will be replaced with new, never before seen high resolution 2D animations drawn by BG:EE's Art Director Nat Jones.
New, high resolution GUI.
The Graphic User Interface (GUI) has been completely unhardcoded, thus making it possible to overcome its inherent limitations in terms of resolution. Beamdog / Overhaul Games are currently working on a brand new, high resolution GUI drawn by BG:EE's Art Director Nat Jones and fellow artist Madoline Bee, which will be customizable / moddable to a generous extent (some limits still apply due to the original BG code's resilience to Cameron Tofer's attacks; hopefully, Cameron will manage to find a silver-coated machete with which to treat the old code post-ship, further expanding the GUI's customizability).
Flipbook animation transparency and Alpha blending.
As per this thread, BG:EE will feature flipbook animation transparency and alpha blending for BAM files. In the longer run, a new version of BAM files with multiple-layer alpha blending is being considered.
One Pixel Production (1PP).
Erephine has agreed to include the 1PP work into BG:EE. The devs are currently trying to make it play nice with the BG:EE setup. Share your opinion on which components would suit BG:EE better here!
AUDIO ENHANCEMENTS
OpenAL based audio engine.
The audio engine has been completely ported to OpenAL, thus ditching the now obsolete DirectSound API, thus bringing back environmental audio support for newer OSs.
Original music by Michael Hoeing.
Purists have nothing to fear, as the original soundtrack by Michael Hoeing will remain untouched in BG:EE.
New music by Sam Hulick.
Sam Hulick, the composer who scored the soundtrack to Mass Effect 3, will write new music to go with the new contents in BG:EE. He is an admittedly huge fan of Michael Hoeing's work, and is trying his best to remain faithful to its spirit and atmosphere.N
NEW CONTENT
One new playable kit.
BG:EE will feature one new playable kit never before been seen in an IE game: the Blackguard.
Three new joinable NPCs.
BG:EE will feature three new joinable NPCs: Rasaad, a human male Monk; Neera, a half-elven female Wild Mage; and Dorn, a half-orc male Blackguard. Their dialogues, romances and interjetions will be written by fantasy novelist Dave Gross, alongside Phillip Daigle. All three characters will be romanceable.
One new adventure.
BG:EE will feature one new adventure with dedicated, newly developed area art and "nasty AI". The Black Pits (codename: Adventure X) is a standalone arena mini-campaign consisting of 15 fights and taking place in the Underdark. New and old NPCs will react to the new content with interjections.
Several new items.
The devs are adding a number of new items to BG:EE to even the item spread out a bit for the classes and the new kits.
New portraits for player characters.
A portrait pack drawn by former Black Isle Studios portrait artist Jason Manley is being added for free to PC/Mac, and as paid DLC to iOS/Android.
New voice sets for player characters.
A new collection of player character voice sets is bundled with BG:EE. It will include three new male and three new female voiced sets. One of the voice actors has been confirmed to have already done voice-over work for the Baldur's Gate saga.