One of the goals of most websites and blogs is to make money. Whether the intention is to build a profitable business or just to cover the costs to operate the site, most website owners need to get something back from their work monetarily to justify the effort that is going into the website.
Unfortunately, sometimes monetizing a website comes at the expense of its readers and visitors. Obviously, in order to maintain a successful web presence, the site must be monetized in a way that is acceptable to visitors.
I see these issues as being keys to monetizing a website without turning off your visitors (in no particular order):