I really, REALLY like how masculine you made their features. It makes them seem more...intense or something, I don't know. Really good job though, can't wait to see the finished product. I'll keep the look out for it.
I love the great action poses and the guns which are perfect, but those faces look absolutely nothing like Sam and Dean. If it wasn't for the hair and the group I found this in I wouldn't know who they were.
I think the best way to acheive likeness is to collect some pictures of the characters and outline the shapes of their faces including eyes, nose, and mouth, this will help you see the details that make the persons face look like them. You can also look at other drawing people have made of the characters. I happen to have a collection of drawings of these two that might help. [link][link][link][link][link][link] I hope these can help.