Call it well-developed banner blindness. Too well-developed, I'd say, as I caught myself at times (and ended up in an IRC flamewar once because of it) missing pieces of content because they looked too much like ads. It's amazing how your brain can learn to filter out the noise before it reaches conscious level. I also don't care that much about security implications other than malware. All that combined with me a) generally not visiting mainstream sites, and b) being too lazy to install a new extension meant that I managed to run without ad blocker for a few years.
Ultimately it was YouTube ads that play before videos that were the last straw; I sometimes run YT as music source and I got too pissed off about having to listen to 30 seconds of unskippable advertising in order to get to 120 - 240-seconds long video. Mostly it's mood-breaking.
Amazing you held out that long. The difference between 'with ads' and 'without ads' is so large I can't understand how people in the tech scene (for who it is trivial to set this up, luckily for the advertising industry not all that trivial for others but getting easier all the time) still browse without some kind of blocking. Besides the security implications of running third party code on just about every website.