Most notably, perhaps, it represented Magics first block (or one large set with two related smaller sets following it), which dominated the design and play of Magic for over 20 years
You choose whether or not to cast the target instant or sorcery card as the reflexive triggered ability resolves
5/4 Green is generally the best at hating on artifacts and enchantments, but the possibility of reliable, repeatable, and protected removal
The cheap counterspell finds its way into many decks and is effective with only a couple copies, not to mention it's super easy to add to your collection at common
You can only do it once a turn, but its still a big win
Spirits, as a creature type, keep on getting more and more support every year, differentiating and consolidating a lot of sub-archetypes within this kindred strategy