The WORST. No, maybe not worst. But the nastiest, most insidious, thing I experience on the web must be sites that require javascript to _fully_ load. Not just "you need javascript". But "here you go, all the HTML elements you need, table layout and whatnot - but most of them are PLACEHOLDERS". Wtf?!
This happens on multiple sites where there is a long list of elements that need rendering or hotlinking. If we would do this on !GNUsocial, we'd maybe have the first 2-3 conversations in your home timeline filled out. Then grey boxes. And these boxes would require javascript to load, and for you to navigate to the next page etc. etc.
This phenomenon appears to be present on evil sites like #YouTube (some images load, the rest are left for dead) and #Facebook (you get the table structure, but only a very bad image representation of fake text in the center column).