I've been playing Warcraft Classic for a few days now when I have time, and I've finally sought out an add-on, a mana bar that shows when my druid is shape-shifted. I'm pretty sure this was also the first add-on I installed when I first started playing Warcraft. If I was asked what my first add-on was prior to Warcraft Classic I don't think I would have been able to answer. Such a silly little thing that eventually got baked into the standard UI and the add-on forgotten, but at the same time such important information to have.
Now I've only gone so far as to find and download the add-on so far, DruidBarClassic. I still need to install it and make sure it works next time I play, but I'm pretty optimistic.
We'll see if this leads to another cascade of ever increasing numbers of add-ons, or if my current minimalist outlook on add-ons will prevail. A lot of that may be up to add-on developers since the availability of add-ons is pretty limited as compared to when I started playing near the beginning of the Burning Crusade.