What are people's feelings and opinions about objects that extend horizontally across an image - like a bridge. Should the object be level with the horizon?

What about foreground and backgrounds? Should the subject definitely be in either the immediate foreground or perhaps background (such as mountains) to make it stand out?

On a bit of the same note. Is it considered bad form or too distracting to have bushes, trees, or tall grass in the foreground?

I know they are somewhat vague questions and that "everything is subjective" but I would appreciate opinions.

Thank you.