Don´t run away 
- here are some of my suggestions.
Firstoff, I really recommend to turn the arms downwards, else you´ll never be happy. Just make it seethrough, select the arms with the <b> tool, rotate around 3d cursor, set it at the shoulder joint and use the proportional edit tool to rotate it down and start the retopo.
The area where the the deltoid muscle runs into the trapezius muscle will always be jolted in this pose and overstreched when skinning the mesh and moving the arm down. For the armpit it will be vice versa.
Skinning the shoulders is enough trouble with a good topo even if you use additional deformation bones, your initial setup just makes it harder.
It is a matter of taste though, maybe even religion if you spread the arms wide, or just raise them enough to have the shoulder and armpit deform averagly the same.
Most important is, you don´t have “the flow”
Your loops are all zig-zag and hard… not smooth, no flow.
You got to get yourself to see the flow, which involves a certain amount of anatomical knowledge so you see the muscles even when they are not there.
For the neck area… you didn’t change it at all for what i see, you just made 2 huge quads in the chestspine area.
What I meant was to lengthen the definition of the spine up to the atlas (the tight U-turn at the smileyface arrows hehe)
The back of your skull is box-ish, there is no flow.
The red stuff on the right side, those loops have no real reason, they run the way they do for no reason and the long one limits the shape of the spinebone and the edge between trapezius and deltoid muscle - and for that its too far away from the spine, giving him puny looking bladebones.
The area with the huge triangleshaped quads, is supposed to be where the trapezius muscle runs dow. The base of the muscle is at the head towards the spine and runs down almost until your pelvis, he is responsible to pull your shoulders up and pull your shoulders back, thats the green curved arrows that go away from the spine towards the bladebone.
I hope this helps you a bit, I know it is a hard task, and I know by now you feel bad about me tearing your mesh to pieces, I´ve been there too =)
You have to think more organic, leave the default cube behind :o) , curve the flow and you´ll soon get rid of the squared paper look of your topo.
And for your back it might be a problem, but maybe you got a friend or brother, or your dad, or your wife, dunnow your age and for the back the sex doesn´t really matter =)
You got humans all around you, watch them.
Personally I enjoy looking at this gore-machine. Sometimes I sit somewhere and just move around one finger and watch what is happenening, where the joints are, what the sinews are doing and on what muscles they might be attached.
Let someone you know do pushups and watch their back, let them stand in front of you and lower and raise their arms, watch the bladebone move and the deltoid work.
Let them raise their shoulders and see how the trapezius moves…
(obviously the person you choose should be rather “normal build” in order to see that stuff)
Of grab your digicam and make a short video of them doing the action once and watch it.
If you feel uncomfy asking or watching half naked people you know, record a body building competition on TV or youtube for them, although bodybuilders are not really specimens of the golden number =) they are made of the same parts than other humans hrhr.
When it comes to human modelling it´s really all anatomy of muscles/sinews, skeleton, skin and fat and understanding how the body is able to move
