Overwatch is NOT a reaction in D&D terms. It would be the equivalent of a D&D ready action.
Yeah, and what ?
Reactions just have other triggers + other effects. The only significant difference is that you're planning their uses during the ennemies turn rather than during yours.
It could be boring but there would not have much differences if you were able to plan as much reactions as you wish (with the targets and triggers) during your turn. You'd just eventually have to toggle your feather fall OFF depending the initiative order. Or you could only prepare a counterspell that trigger against lvl3+ spell if that's your priority...