From a BF hater:
Dice played it safe for BF4, it's what BF3 should have been, but whatever.
The SP is a graphic showcase and boring as hell. The graphics/lightings/weather are really great and they almost avoided the glare/flare effect abuse. The story is really boring and there is a lot of holes in the plot. Basically you cruise around the world with 2-3 teamates and the only order you can give them is "shoot X pack of enemies". But they're so slow/bad at doing the job, you end up being the one-man army taking care of everything and they just tag along for the ride. Honestly I don't know why they still bother with SP, it's just a waste of resources that could have been used somewhere else.
So yeah no surprise there the MP is where things happen.
The MP is a lot like BF3. There is a mix of old and new stuff and a lot of tweaks that makes a huge difference. The level of destruction is still nowhere need Bad Company 2, but it still better than BF3. The maps are well designed and the changing weather (even if it's scripted) is really cool. There is a map that is a bit like Wake Island and after a while a bit storm start washing the island, the sky become dark, there is thunder, rain, strong winds and HUGE waves (when you're driving a boat or a jet ski in the storm, it's one hell of a challenge). Another map is a flooded city and the water level keep rising, really cool too . The weapons/vehicles/kits unlocks were refined a bit, some items were swapped around, other tweaked for the better IMO.
Right now there's a few frustrating bugs, you get random freezes (which cause you to lose the stuff you just unlocked) and crashes.
Overall it's BF3, but better. Nothing huge or groundbreaking, but small change that make a difference. So far I don't regret my purchase, it's fun as hell to play with friends.