I could write for years about good design and bad design for the web.
Things I personally can't stand:

- horizontal scrolling
- splash pages
- not knowing where to direct my focus first
- no white space anywhere
- the little blue underline for links
- things that aren't lined up
- wild colors on professional sites
- too much background detail, not enough content
- purple
- things that blink
- video and audio that autoplays
- things that only work in IE
- more color than b&w in the background
- ads that pop over text and then won't go away