First, online ads just feel much more intrusive than the ads you find in printed media. For newspapers in particular, most ads are black-and-white, and they generally fit between articles pretty well (though newspapers have existed for centuries and people have had a lot of experience trying to balance things out). Online ads are often in blazing color and may be animated, interactive, or make noise, which certainly force you to pay attention, but that drives people toward using the ad blockers more heavily.
I'm not too concerned about network bandwidth these days, but ads can be fairly big files and cause a huge number of connections to a huge variety of web servers. Slow downloads have always driven people away from websites, so there should be a pull toward keeping ads as minimal as possible. That problem may sneak under the radar because the savvier users who would most likely complain are also the ones most likely to use ad blockers.
All that extra included content also brings security risks. Advertising networks are potentially delivery networks for malware, and I've never been very confident in the ad companies' interests in ensuring that the content they serve is safe for consumption. They'll say it's important if you ask, but it's hard to trust someone who has an inherent interest in tracking users and moving ad content through their approval pipeline as quickly as possible.
There's also the problem of ads pretending to be site content -- The thing that really pushed me over the edge into aggressively ad-blocking was when Facebook kept on having ads that included buttons that looked almost exactly like ones that were needed for maneuvering around the website (though the ads usually had green buttons instead of blue ones). Even in print, ads that tried to blend in too well were the ones that tended to bother me the most.
So here are a few of my preferred rules for ads (though some rules would be different for ads within videos):
- Most ads should be text-only or in black-and-white
- No auto-playing video content, and especially nothing that makes sound
- Ad files should be small to keep load times short
- Ads should be delivered from one source (not spread across a multitude of servers)
- No repeating the same ad across multiple spaces on a single page
- Ads must be dissimilar from true news content or other features of the website design
- Ads in videos must take less than, say, 25% of the total play time of the video
- Ads in videos must not prevent use of the pause or mute functions of the video player