Romita and Janson's work on Thor has a lot of surface lines covering every character. Surface lines, when used correctly, can add a lot of depth and form to a drawing. But they can also quickly turn into a mess if you don't use them right.
It sounds obvious, but they should never be added arbitrarily and they should always describe the form they're on. In this example they are drawn so that they wrap around the forms of the muscles. That's how surface lines are used almost all of the time.
In the page above, the large image of Thor at lower left is another example of surface lines used well. They describe both Thor's form and the lighting in the scene very effectively.
In the large horizontal panel below of Thor flying, notice how Romita and Janson use surface lines in a slightly different way. In this case, all the surface lines are drawn in the direction of his movement to accentuate and emphasize the direction that Thor is traveling in.
When surface lines don't wrap around muscles, or emphasize the direction of travel, they're usually drawn to indicate the way the light is falling. Take a look at the first panel in this Jonah Hex page by Jordi Bernet. Most of the lines are drawn in the direction that the sun's rays are falling on the figures (except for a few lines that follow the surface detail of their faces).
The only other way I can think of to use surface lines is that sometimes they are used to indicate weather (like rain or wind). In this Jonah Hex page, all the figures in the second panel are trapped in a dust storm, and all of their surface lines are drawn in the direction of the wind gusts of the storm (this example is actually from "Jonah Hex #59, artwork by Jordi Bernet).
This is a well-known drawing principle, but it bears repeating: the number of surface lines on a face can have a big effect on whether a character looks old or young. The more lines, the older the character tends to look.
Here, to keep Jane Foster looking young and beautiful, Romita uses very few lines...
...in contrast with how he draws the face of the ancient and wizened Odin.
Check out the difference in the way Romita and Janson draw Peter Parker's face and Aunt May's face and how surface lines make a difference in their perceived ages (yes, they both appeared in "The Mighty Thor").
Surface lines can be used to create depth and a sense of space as well. Here, in the right middle panel (below), Romita uses the surface lines of The Destroyer to do that very effectively. It's easy to see which parts of the figure are above us, at our eye level, and below our eye level based on how the surface lines curve.
Simple vs. Complex
One basic drawing concept that I feel never gets expressed enough is that you should always put complicated areas of a drawing next to simple areas. This creates contrast and visual interest, and is the only way to make complicated areas work. So many times I see drawings where the opposite is true: complicated areas next to complicated areas, and simple areas next to simple areas. That's just not interesting and often creates visual confusion.
Here, the flames in front of The Destroyer create empty, blank flat areas that contrast nicely with all the reflections and texture on his surface. as well as the complicated background texture and the splintered wood at his feet. This technique creates depth and visual interest, and it keeps the drawing from becoming a complicated mess where everything has an equal amount of busy texture and visual emphasis.
In the top panel of the page below, more of that idea on display: the simple flames in the foreground contrast well with all the complicated little figures in the background and create some nice depth.
A simple panel (that I cropped from the rest of the page) showing how the wrinkles and creases on the back of the delivery man contrast well with the blank areas of his outfit. Notice how all the lines wrap around his form as well.
It works really well to create a layout with layers of complicated background structures separated by layers of a simpler form, say, trees and foliage, or, in this case, smoke.
The effect of complex layers (of architecture, usually) interspersed with simpler layers (of smoke or foliage, usually) can create an effective sense of space and depth, and it's much easier to draw than, say, a complicated bunch of buildings receding back in perspective.