I'd say BF4 is what BF3 should have been. It also goes a good middle way of implementing previous features that have been missing or swapped in BC2/BF3. Examples:
Commander mode is back.
Both recon and support can aquire C4 (was only for recon in BC2 and support only in BF3)
Engineer and recon have additional weapon classes, carbine and DMR, that once unlocked can be used freely by other classes. This in addition to shotgun gives you 4 usable weapon classes per class. In total: Assault rifles, Light Machine Guns, Sniper Rifles, Personal Defence Weapons, Carbines, Designated Marksman Rifles, Shotguns, Pistols, Gadget/AT.
Weapon class unlocks and class unlocks are separate. You mainly unlock weapons by using that specific weapon class, while you unlock mostly gadgets with your class.
Your two gadget slots can use any combination (unless marked red), instead of only using certain gadgets in slot 1, and certain gadgets in slot 2.
Each weapon has lots of attachments, which are earned per weapon. Most attachments are available in US, Russian and Chinese variants, if you prefer one country's aesthetics over another.
More variety in how you obtain unlocks, some through class progress, others from battlepacks, or from completing certain objectives.
Generally summed up: More freedom for class customisation.
Other features:
More vertical maps. Most buildings and sniper nests have elevators now, so you can sneak up on those pesky bastards.
Water is more than just an obstacle. You can swim underwater, waves can be used as visual cover, also basic use of sidearm or gadgets.
Levolution, sometimes cool, other times useless, but there's also more world interaction, you can close and open doors, activate traps and so forth.
You get bonus points for trying but failing, like: "Assist counts as kill" is a nice addition if you're that guy who always lowers enemies health to 5% before a teammate steals the kill. You get points for doing objectives a certain percentage (if you only armed an objective 65%), you still get points for the effort. This gives at least some reward and that kick to keep you playing, instead of failing at 90% again and again, and not getting any reward at all.
I probably have some more, but I've forgotten them right now.
"Bad" points:
Anti-Air is actually Anti-Everything. Combat boats are also slightly OP.
"Net-Code", personally I haven't noticed much of this. But everyone seems to be screaming it all the time.
Although there are lots of new weapons, some previous weapons have disappeared, and will most likely be re-featured in future DLC as "Brand new weapons" or "Old player favourites".
The core gameplay is mostly the same, but they've revamped and rebalanced a lot of the way classes and weapons work. Also added a lot of things that felt like they were missing in BF3.
