VoIP in game for PC, for one.
Commander is back
Levolution
Netcode improved -- dying behind cover happens much less. Still not as good as it could be, but a definite improvement.
Colors are warmer -- less harsh blues, more warm oranges.
Next gen 64player console support.
3 factions, and you can switch any of those factions on any map (example: you could play Siege of Shanghai as US vs RU or RU vs CH or CH vs US)
Considering how good the game looks, it can run on a lot of medium computers, possibly even low end (but I haven't checked)
Dynamic weather
There is more microdestruction and building destruction -- it's in between Bf3 and BC2.
Physical waves that sync in MP
Vehicles have more momentum, and are easier to pilot/drive for the most part.
Lights and how they work in general improved -- volumetric lighting, shadows, etc.
Battlelog 2.0 allows you to change loadout on the fly, have a second monitor/tablet show the minimap, and more features.
Ability to connect to commander on tablets.
Much more detail in the environment (props, textures, etc)
The player has a lot more to do in terms of changing the map with levolution.
Canted ironsights and some other attachments from MOH.
IRNV now has a range more than 5m, lol.
Gunplay feels a bit heavier, while still being kinda fast paced because of the mini medicbags and such.
The ability to zero your scope
Rangefinder
Wire guided launchers now ingame (EX: SRAW)
Some weapons that were lock on before are now requiring to hold your lock, instead of fire and forget. The ones that are fire and forget do less damage.
Attack boats added, jetskis added, rhib boats actually useful. Swimming is faster now, and you can knife and pull out your secondary in the water.
Animations are smoother, look a bit more realistic.
Better, more precise callouts (instead of "Uhh we've spotted an enemy assault, over" it actually says "Enemy soldier spotted West of your position")
Voice acting has improved a lot. From the russianglish in BF3 to actual Russian and Chinese in BF4.
Sun, taclight and laser blinding abilities toned down.
You can now do a quick revive for 20 hp and then charge for 100 hp
Field Upgrades replacing perks, only able to progress by squad play.
Defuse and Obliteration -- Defuse is going to be good for e-Sports, while Obliteration is something I've never seen in any game before. Correct me if I'm wrong.
Some of the dials and systems in jets and helicopters actually light up and such, and the FOV in jets is no longer annoying like in BF3. (raised higher)
Ragdoll physics are improved over BF3 by a lot.
Adjustable magnification.
You can spot only by pressing Q now, or if you have a motion detector. When you press Q, you begin a small animation that points at what you spotted. It doesn't interrupt fire.
Sounds are more crisp and more authentic. Guns in BF3 still sounded a little weak, but in BF4 they sound very strong.
Suppressors actually sound like suppressors now.
Camos for every single weapon and vehicle now, and your main platoon emblem now gets painted onto your vehicle.
UI completely overhauled - especially spawn and customization. Just in general easier to navigate and better looking.
Spectator mode
Killcam reworked.
Attachment system completely changed.
Ability to see your weapon stats ingame and they update when you put various attachments on them.
Incindiary, flash and normal grenades available.
More ways to get points (Catastrophic kills, dogtag payback, etc)
Complete overhaul to the suppression system.
Recon and support both get C4 and claymores now
Mortars are getting a complete overhaul.
Minimap system changed.
Battlepack system introduced.
New models for basically everything.
Volumetric lighting introduced.
Working elevators (atm it just teleports you after waiting a little bit, but it's progress.)
Micro destruction much much improved.
Reflection system much improved. Glass shards reflect the world around them now, even.
More esports support now.
Maps for every type of player. Huge open maps, smaller linear ones and everything in between with every type of environment you can think of.
Jets get JDAMs
Attack jets get TV missiles
Mobile artillery completely reworked. Much easier to use; involves picture in picture.
Helicopters now spin out after being destroyed.
Commander mode assets can change from map to map. Some have a submarine, some don't.
Scout helicopters can change what type of minigun ammo they have (ex: 25mm)
Official servers, where it's DICE settings and admins can't kick/ban/etc. If you use quick match, these are the servers you join.
Stationary MG emplacements in game.
Dynamic smoke
Tons of new ways to get points thru teamplay.
Mini medic bags and ammo bags help speed up firefights without speeding up the pace of the game too much.
Commander joins thru battlelog, and isn't an actual person in game.
Counterknifing introduced, as well as just a general overhaul of the knifing system.
Supply kits dropped by commanders allow you to change your kit.
New gamemodes to appeal to everyone.
Carbines, DMRs, shotguns are all kit weapons now.
Engineer now has PDWs.
Driving is a lot better, tanks especially. A bit more grip, and they can actually drive through obstacles like cars and barricades.
More options for colorblind mode.
Able to chat in end round screen.
Battlelog in game allows you to easily set up missions for your friends, join off of friends in game, and more.
Tac lights can no longer be toggled, but turn on automatically when you aim down sights.
In response to above, DICE added a flashlight that works like the BF3 one, but far less blinding.
Tessellation was improved drastically.
Color palette changed from a gritty, blue and dark one to more vibrant and colorful, and more realistic one.
All them maps have intense verticallity to them. As in, every map isn't just flat with 1 story buildings, but actually has a lot of accessible layers and rooms etc above ground.
New types of grenades, includes impact, incendiary, smoke, flash, frag, and more.
Leaning introduced.
Vehicle damage reworked completely -- can now have their mobility targeted.
Maps in general seem larger than BF3, and are much more well designed. More alleys, microdestruction, levolution, etc.
Ability to change loadout in campaign.
How they patch is much different now. Gun balancing, IIRC, is server side, so they can release a patch for balancing right away. Also, Sony and MS changed their policies on patching, so more patching
Tank gunners have seperate loadout from drivers, and can carry defensive items (ex: incendiary grenade)
Large increase to countermeasure timers.
Flares now attract locks as well as break them.
The tops of vehicles now have their own damage sections, since verticallity is such a big thing.
Tiered reloading introduced.
Game now shows what direction you're getting locked from.
Ladders reworked; can sprint up them now, and you can now look around while climbing every ladder.
MP-APS introduced. Support can place it on the ground, and a very small zone is protected from all explosions as long as it isn't reloading.