Lack of contrast and having a variegated or "busy" background behind the text makes me nuts.
Also text that is too small and can't be resized. For example, the street names on Google Maps on my phone, if I zoom in, the map size changes but he font size of the street names does not.
@Leana @Anna Yes, that. Also gratuitous moving objects, especially in the periphery of my vision (sometimes I have to obscure those with my hand to read the text). Being interrupted by a popup in the middle of reading something.