+1 to Composer's feedback screenshot. It basically exactly matches with my opinions on reactions:
- We should have the option for full flexibility of reactions, being given the choice to use them or not instead of them being used automatically or according to some preset script.
- It doesn't *need* to be a central pop-up window. That is just a stand-in term for being asked, but I can see how that term has certain implications and people wouldn't want a pop-up in their face. Having a mini-hotbar show up (one might say, pop up :P) below with available reactions, along with whatever slowing-down/cinematic effects are applied to combat, would be great (also credit @GM4Him).
- less clicks = better

In your examples, I might specify that "Wizards should have the ability to counter specific spells, rather than e.g., automatically counterspelling the first spell cast at them (possibly a cantrip)"