The big change which will come with ETIAS (or rather, the overall Smart Borders Package program of which ETIAS is a part) is that you will no longer be able to slide through Schengen overstays.
There will be a centralized database for the first time which will electronically register every entry and departure from Schengen, instantly flagging overstays.
Enforcement in the past was quite spotty, based on laborious checks of passport stamps. No longer. There will no longer even be passport stamps.
The preclearance procedure itself seems fairly trivial; just do it well in advance.
The big unanswered question is how this will
work in connection with the bilateral agreements the U.S. and some other countries have, which give longer than Schengen's 90/180 in several countries. The problem is no borders and no checks within Schengen. Some information suggests that you will now have to actually apply for bilateral treaty benefits, which will then be put into your file in the system. But how will they know you spent that time, in that country? These bilateral agreements are not quite harmonious with the idea of Schengen and it will be interesting to see if they survive.