There are just too many spells and abilities that are dependant on reactions working as they should, because having them trigger on the first enemy action rather than an action you *want* them to be triggered by (and, as such, saving your reaction for when it matters the most) is kind of the whole point and the keystone feature of the 5e combat. At this rate, the only reaction in the game would be the opportunity attack, which just turns it into a bleeding stump of a system that was never properly implemented.
Counterspell, mentioned above, is probably the most shining example of how it depends heavily on getting the system right. Why would one want to waste it on a magic missile when some other enemy just gets to cast a fireball, scot-free? They *have* to get them right, or the game would definitely suffer for it...
