For one, suppression. In BF4, suppression no longer anal rapes you and then fornicates your eye socket. This makes defense against the spray and pray people possible, suppression for Recon in BF3 was absolutely game breaking. you can have a perfect headshot, but because you're suppressed *Insertnope.jpg*
Second, all weapons feel much more lethal and more accurate (probably due to the nerfs/adjustments to suppression). Vehicles also require strategy to play well and to stay alive (they also appear to be more durable)
DMR's are actually very useful now, they actually feel good to use, minus the weird hit reg issues on moving targets.
the XM25 makes camping or enemies hiding behind cover a non issue, provided they're stupid enough to stay near the edges.
General movement of the character and the vehicles have weight to them. Guns feel meaty and satisfying to shoot, no longer do you feel like you're firing a fancy prop item that's made of plastic.
there are more ways to counter armor especially aircraft (at least when helicopters are concerned, can't say about jets)
The list would go on forever, but for the TL;DR people
BF4 IN A NUTSHELL:
BF4 >>>>>>>>>>>>>>>>>>>>>>>>>>>>>> BF3