Other than occasionally having my toon automatically swing for an attack of opportunity I really couldn't say what other options are here for reactions. However, if players see popups as a problem they could also slow down time, similar to the slow turn based movement they already have, and give a chance to use a reaction that is in your list. Combat right now feels very static. I go...enemy goes and I wait 5 minutes for the 10 goblins to move in a row. Reactions are a great way to break this up and make it feel more alive. I just don't think they have implemented enough of them in a useful way so far. Maybe I'm missing something though so if you have a different experience other than some auto attack feature please let me know what else there is as far as the reaction system goes.
The ones available are mostly class features or certain abilities you can get. Example: Light Cleric have warding flare, Fighter Battlemaster have riposte, Warlock have Hellish Rebuke.
They haven't implemented everything, as they are testing the current implementation to see how well it goes. And there will be issues later, like warding flare can be applied to other party member at level 8.