Too much text Some people put far too much text into their pages, and often don't have margins or breaks between paragraphs. The result is a screen full of text - which is difficult to read. Use plenty of space around your text. It will look more attractive and be easier on the eye.
Bad colours Combining coloured backgrounds with coloured text is dangerous.
It can work if it is done with restraint - but many people are attracted to colours which clash and vibrate. For good results - 'less is more' is often the case so far as color is concerned. Tiled backgrounds Background graphics can often make text difficult to read. When a .gif file is tiled across a page, the resulting pattern often creates an effect which is visually disruptive. Use only very pale graphics.