The main reason I block ads is because ad networks make so many requests (and the ads load so slow) that it always affects my browsing. I am either waiting for the page to load because of poorly engineered synchronous ad network code, or I'm having my content fumbled around at the tail end of pageload. Both situations are obtrusive, aggravating, and slow me down.

In the early 2000s ads were simple images that loaded just as fast as everything else, and I didn't mind them much, even the cheesy animated ones. It's the delivery, not the content, that I abhor.