These things are over the line (and illegal):
Using images without permission Using code without permission Referencing images and code from another site without permission
Designing a site from scratch to roughly copy another site's design
This is not illegal, merely poor form and also a bad technical decision because the other site can then control you.
If this were illegal, you would have to get permission from other sites to use iframes. Things like stumbleupon's web version couldn't exist.
These things are over the line (and illegal):
Gray area: