I paste the (possibily primarily?) changelog here. I removed some changes which I consider to be spoilers. It is safe to say that a lot of new monsters and creatures have been added.
Random characters have professions applied.
Martial arts techniques can trigger when wielding appropriate weapons.
Wing mirrors for vehicles.
Adjust zombie speed with mods.
Made butcher menu's first entry match the butcher hotkey, so you can just hold down the button to butcher all.
You can dig down stairs with proper equipment.
Scabbards, holsters and sheathable weapons.
At high skills you can quick draw/attack with weapons.
Fish spawn as distinct creatures, and they're used for fishing checks.
Quivers for arrows.
Furniture can act as a tool.
Corpses rot and eventually disappear.
Armor is assigned to a layer, and armor on different layers doesn't impose layering penalties.
Destroyed vehicle parts get ripped off the vehicle.
Player can start at various locations.
Special support for mapsharing installs:
lock down debug menu
lock players to their username
more configurable file placement
locks around map files
Language switch is dynamic now.
Guns such as revolvers no longer eject casings when fired, they're instead ejected on reload.
Spending more time near traps has incremental chance of finding them, spotted traps are remembered.
Thrown items above a trap-defined threshold will trigger traps.
You can now peek through curtains and some doors.
Smashing a broken window again clears out the broken glass (relatively quietly).
Autopickup can get at anything you can reach, i.e. adjacent tiles.
Can set exclusion zones for autopickup, so you can avoid accidentally picking up all your stuff in your base.
Contents of books are unknown until first read (which is faster than usual).
Can craft recipes if you have a book in range that provides it.
Can learn recipes by practicing them.
Roof of the vehicle you're currently in is not drawn.
Faction hostility is back.
Kiln operates without player being nearby.
Radio operated items.
Switched to seed-based weather generation.
Options are only displayed when they are applicable to your build.
Can perform unspeakable acts upon zombies to turn them into your packmule slave thing.
Messages give feedback on melee misses.
Foraging is seasonal, fruits will grow each season, but once picked are gone until the next.
Pet menu to interact with friendly monsters.
Starting scenarios stitch together profession, traits, and starting location.
Picking up and dropping multiple items is now an interruptable activity.
Fire creates invisible hot air fields, which meander around in enclosed spaces and warm them up.
Factored windchill and humidity into body temperature calculations.
Vehicles smash items when they (wheels) run over them.
Refined vehicle/terrain collisions.
Jumper cables can be used to link together electrical systems of vehicles.
Crafting a large batch of items can apply a discount to the crafting time required.
Overhaul of "cut up" system:
Cannot cut up an item if it includes a material that can't be cut up (e.g. shotgun).
Cutting up returns items from each material that makes up the item.
Cuting up most items should respect the total volume of the item.
Most terrain can be smashed now, if you can hit it hard enough.
Switched to SDL 2.0.
Made paths configurable for packaging and installation support.
Finished pushing martial art definitions out to json.
JSONized multi-tile building generation, leading to huger buildings.
Tilesets can provide a per-category fallback tile to handle new monsters/items/whatever.
Silenced all the warnings, now it's easier to keep new ones from cropping up.
Lots of string handling infrastructure, making translated string handling easier.
Missing tiles in tilesets can fall back to ASCII.
Saner glyph cache handling.
Components used to craft an item are stored in the item.
Debug overlay for hordes.
Improved window handling.
Generic iuse functors unify many types of iuse functions.
JSONized bullet pulling recipes.
Generic support for foldable vehicles, still need support for *crafting* them.
Encapsulation applied to map accessors.
JSONized monster drops.
Damaged fuel tanks leak fuel on the ground.
Most menus use input context now, and therefore have configurable keybindings.
Switched build to c++11.
Items can have a list of iuse methods instead of just one.
Drug effects can mostly be defined in JSON.
Scripts for querying json in tools_json_tools.
Beefed up utf-8 input handling.
JSONized NPC definitions.
Unified recipe requirements across constructions and crafting.
Better error handling when initializing SDL.
Recipes can have byproducts.
Added a standalone map layout utility.
Monsters are saved to the overmap instead of per-submap, clearing the way for horde support.
Hgamelaunch start script.
Items can spawn with variable amounts of charge.
Monster blacklists are applied consistently to all spawn code.
Different ammunition-bearing monsters (turrets, robots) have customized loadouts of their various ammo types.
Give most starting professions decent starting equipment.
Capped skill gain from many activities, especially crafting.
Melee weapons can take wear and tear damage based on their primary material.
Categorized CBMs you can get from butchering things.
Armor values nerfed.
Mutagen spawn rates slashed.
Overhauled mp3 player morale bonus to make micromanagement of it suboptimal.
Amount of damage blocked by a block technique based on strength and skill of blocker.
Removed acid rain until it can be made into a local event.
Nerfed speed of many zombie animals.
Starting professions supplied with cold-weather gear to match the expected climate.
Standing in a fire less likely to ignite clothes.
Lying in fire (when one or more legs is at 0HP) causes MUCH more damage.
Hops, barley, and molasses as brewing ingredients.
Yeast for fermenting brews and baking some bread foods.
Sugar beet as a renewable source of sugar.
Fruit wine and Brandy.
Moonshine and home-brewed beer.
Many new house layouts.
Wells and well digging.
Decorative terrain and furniture.
Parasites and diseases from unsafe foods.
Curtains for vehicle doors and windshields, privacy please!
Foraging (in bushes, shrubs, trees).
Towel usable for many handy things (still not at HHGttG level).
Wearable rx12 auto-injector.
Killed bots drop (disassemblable) broken bot items instead of corpses.
Nice looking road ends and roundabouts.
Pizza parlor building.
Several toolbox items to streamline carrying around your crafting needfulls.
Cotton and many cotton-based recipes and items.
Audit of all item prices to try and get a sensical baseline for them.
Make Fedora activatable, m'ilady.
Rollerblades and skates.
RC car and remote-detonated bombs, and a remote controlled lamp.
Automated gas stations.
E-ink tablet and laptops can display recipes and act like books.
Can take pictures of monsters and view them on laptops or tablets.
Various power substations.
Diesel added as a vehicle fuel.
Added craftable pontoon bridges.
Expanded armor layering window.
Added way to swap panes in AIM.
Animation and duration for smoking activity.
Action menu that displays all actions in a tree menu.
Highlight things you can interact with when 'e'xamining.
Many strings are properly pluralized and have support for doing so when translated as well.
Standardized overmap building colors.
Vehicle examine menu shows used/capacity of fuel tanks and cargo space.
SDL builds can play music.
Log messages colorized.
Books colorized by category.
Different weather types get their own animations.
Save/load log messages.
Fuzzy river borders.
Scrolling text animations when things are damaged and on some effects triggering.
Lots of menus now have sorting, categories, and search.
Animation delay option.
Player can write and read sign items.
Added a menu displaying martial art style characteristics.
Perishable and rotting items colorized.
UPS no longer activatable, now act as passive batteries.
Greatly sped up construction menu.
Overmap scrolling speed greatly improved.
Turn number added to calendar since it's called a gajillion times.
Switched a lot of large collections to use std::unordered_map() or std::unordered_set().
SDL framebuffer cache.
Extracted rain animation code from main drawing code.
No bleeding effect if your armor absorbs all the damage.
Make game stop treating trees, walls, etc as hallucinatory with respect to vehicle collisions.
Prevent dragging furniture from hoovering up items.
Option to force software rendering to work around hardware support issues.
Players are ejected from vehicles when their seats are destroyed.
Contents of items destroyed by interacting with them are dumped instead of being deleted.
Restored function of area melee attacks.
Made various battery mods play nice together.
Prevent segfault in the unlikely case that you miss so badly that you shoot yourself.
Don't cause blood spatters when the attack does no damage.
Avoid crashing when loading a corrupted submap.
Avoid resetting creature speed to 100 when a creature is saved/loaded.
Option to disable joystick input in case you have a broken joystick.
Always practice at least one of the melee skill types when attacking.
Fixed off-by-one error that made 100% coverage armor 99% coverage.
Move critical multiplier calculaton after armor absorbs damage.
Added move cost for interacting with computers.
Use maximum heard sound for effects (such as deafness), not the sum.
Player graveyard works as intended, most recent player save is moved to the graveyard folder.
Factored weight of items in furniture into cost of dragging the furniture.
Gunmods with firing modes finally make correct sounds when fired.