Readability
For most blogs, people come to 'read'. They don't come to look at ads popping up here and there, especially not under their cursor, nor a full-screen terror.
If you are one of theses ad abusers, stop right now. You have no respect for your readers, nor your site. There are plenty of other ways to do ads, and do them right.
Again, people come to read. You don't want your lovely readers squinting at grey-on black texts or magnifying their viewport every time they visit your site.
Other than content, you should make readability the top priority. Accessibility often comes together, as long as you follow web standards.