Here are my definitions:
accessibility - works on all browsers, doesn't use weird fonts
structure - use a normal layout that people are used to, use a site map
links - check for broken links, make sure they go where you intend (I copied and pasted a link to a cute song onetime and posted it to a forum. I didn't check it before posting. When it was clicked on it went to a disgusting picture of a nude fat man. ALWAYS check your links!)