Wings differ from BCD's mainly in that the air cell is all behind you rather than wrapped around you. This allolws a very big air bladder - more lift for when you might be loaded up with multiple bottles for deep dives etc. Also it leaves the chest and under-arm area uncluttered for ease of movement. The system tends to incorporate a solid (usually metal) backplate and harness made of tough nylon straps and buckles. This is much more robust and durable than a BCD which is important because you might be hanging lots of weight off it. The wings system is modular; twinning bands-backplat-wing-harness. Allows you to set it up just how you like it.On the downside wings don't provide a natural heads up postion on the surface when inflated - it takes practice and conciousness to float face up in a wing! And wings don't have pockets. Other than that they both essentially do the same job.
Dom's site is excellent for explaining the issue in question.There's acouple of things too that I think I can add to the thing about BCD's(not being picky Dom,just a couple of thoughts
There's actually many BCD's that won't hold a diver face up on the surface either!When BCD's began to be used over the older ABLJ's many complaints were made re.this.Much the same as the wings arguments.At one time much of the diving gear in the UK had a BS stamp,most BCD's failed to attain this.
It was an issue that a bouyancy device hold one face up on the surface,you may well have an unconcious casualty,or even have to send one up unattended.A long wait,esp in choppy seas for a boat pickup is enough for people to be thankfull that their position was face up and comfy.
Wings grew out of tech/cave arenas where trim with more weight,stages etc.than the normal rec.diver dictated their need.Their intended purpose was slightly different to BCD's etc in many cases(in a cave you don't need something that will lift you up in an incident,all you'll do is bang your head!)
As you say though,they are naturally much more comfy and natural than BCD's.There are wings that will lift,kitted divers and keep their faces out of the water on the surface(I really think this is a very important point,especially with novices who may surface flustered,casualties needing a tow or EAR,the points above,ETC.)
I think it's the same as with a lot of gear,one needs to know what's available and what they actually require a piece of kit to do before making a (hopefully informed choice),this is the hard thing however.Sometimes what we need is'nt what we want,with many divers they're usually blissfully unaware actually what they need!Then they need to be sure that the chosen kit will fulfill it's intened use,much of it won't.Sites like Dom's can go a long way in helping people acheive this.