The "No, thanks" window
The unexpected appearance of a modal window on a website that has a small “No, thanks” link that is almost invisible and the only way to close the window and return to the main content of the site, is a case for design. Before any random visitors can become users, they need time to understand the content and the mechanisms to move around. They need to be fully concentrated on what's in front of them, even when only for a short time, so we can't permit any modal windows to get in the way of reaching this state of mind. We should enable people to extract maximum value from the site in their allocated time with the least effort possible. We simply can't do this if we interrupt constantly their actions and break the overall user experience.
Product announcements and offerings must be part of the body of the site with visitors having the choice to pick what best suits their needs. We need to ensure that we go out of their way and let them determine what is right. Using modal windows is a push strategy, with the desire to control at its root. But design must be there to lead the eye and guide the right behavior, not to control it. There's something democratic in the pull and something totalitarian in the push. Only we can choose on which side we want to be.
The modal window has the hidden “No, thanks” in itself, because the desire to promote leads to more resentment towards the promotions. The offer will be then perceived of lower quality, because it's presentation was bad. The experience of the product is never only the product itself, but everything around it. The value of the product then might be determined by the weakest aspect around it, and this can be everything in the range from design to shipping and communication. The use of modal windows that are hard to close practically says that the presentational aspect isn't important to us. No amount of marketing can offset the effect on perception that the worst product aspect has.
Disallowing choice through making important links hard to notice means that future offerings on the site will need even bigger sales efforts, since all users have now warned their contacts of the potential pitfalls of using the site. Internet remembers everything and this can support or hurt site owners who follow bad practices—long after the fact.
If the design relies on people to have installed ad blockers, it's not right. The problem is then exacerbated when users start to accuse each other of not having installed such a blocker, when in reality this shouldn't have been made their problem. Designers can't simply submit work to their users this way. Any application that works on top of the browser and is required for the optimal operation of a site is a dependency, which constrains what users will be able to perceive from the vast pool of information. The desire to block one bad modal window once will then lead to blocking all other windows, entirely excluding the possibility that some of them might be designed well in the future. Good design eliminates assumptions and dependencies completely. If not, "No, thanks".