Imagine it like this. I can do unlimited Reactions. I have Counterspell. 3 booyaghs in battle. They each cast a spell. I'm out of level 3 spells. No choice. They only cast Bless, but I wasted my level 3 spells on it because of unlimited Reactions.
To make it work, we'd need unlimited spell slots too, but then no one would EVER cast a spell again, for Counterspell could then stop every one every time because of unlimited Reactions and spell slots.
In fairness, I've already put forward my opinion that Counterspell shouldn't be a reaction.
In my opinion, it should be a readied action that burns up your action. Ready versus spell to counterspell. If someone begins a spell, they are interrupted, and it's now your turn to try your counterspell. If no one begins a spell then you wasted your turn keeping an eye out for one.
Oh man. That would suck. Counterspell being used as an Action? To me, that's far worse than having a Potion as an Action. Who would ever use it? With it being a Reaction, you have the strategic advantage over spellcasters who don't have it, which is a lot of the standard enemies in the manuals. Pretty much Mage and Archimage are the main enemies you find with Counterspell.
But imagine the round. Gale has Counterspell. He Readies it just in case an enemy uses a powerful spell. This means he gets no action at all and can't cast his own powerful spells just because the enemy MIGHT use a powerful spell. Enemy goes and casts Mirror Image, so Gale gave up his whole turn for nothing.
Or... Gale casts Fireball on his turn and obliterates a bunch of minions. Enemy mage doesn't have Counterspell and couldn't stop him. Enemy goes and casts Mirror Image. Gale didn't waste his turn and was a valuable member of the team instead of a boring punk sitting there just waiting for a serious maybe. Yeah. It'd suck playing a mage like that.
Even IF the enemy mage casts a higher spell, sure, everyone is happy then, but the mage didn't get to really do anything fun and cool like blow up a bunch of minions. He had to simply sit there and actually hope the enemy casts a powerful spell so he feels validated in his choice.
If Counterspell is a Reaction, though, the wizard can still cast Fireball AND save the team from getting fireballed in return. Quite a bit more of a strategic difference. The mage now feels much more validated for having taken Counterspell to begin with.
I'd honestly never use Counterspell if it took an action. I'd rather hit hard and hope I somehow survive the CHANCE of getting hit hard in return.
