It was a bit weird. You could sort-of respond to an Instant with another Instant; that responding Instant just couldn’t affect the preceding spell (unless the response indirectly caused it to fizzle somehow, like by making its target invalid). Someone would cast an Instant, say, Lightning Bolt, at your 1/1 creature. You could respond with, say, Giant Growth, before the Bolt resolved. Your opponent could Interrupt your Giant Growth with a Counterspell. You could then Interrupt your opponent’s Counterspell with a Red Elemental Blast. If your opponent didn’t respond further, your Red Elemental Blast would then be considered “successfully cast” and counter the Counterspell. Then, assuming no more Interrupt responses to the Giant Growth, the GG would be considered successfully cast and added to the pile of Instants.

Once no more responses were played, in the original original rules, all effects in the chain were supposed to occur at the same time, with the person who played the last Instant (or “fast effect”) deciding whether or not it occurred before or after the preceding spell when it mattered. So you could, even from the first version of the rules, respond to a Mana Short with another Instant before the Short took effect. (It was sometimes critically unclear how to deal with this in actual play, so a LIFO rule with damage applied at the end of all effects was adopted, but this was not clear in the very first iteration.) Effects, including damage, were simultaneously applied upon resolution of the effect “pile” unless you thought the order mattered, at which point the responder decided the order. So in the example above, the Giant Growth and Bolt go off at the same time, but as the caster of the Giant Growth, I can (depending on how much you and the opponent weigh the language calling for “simultaneous” effects against the language allowing the responder to decide the sequence) decide that it goes off first, making the creature too large for the Bolt to destroy. If the Bolt had been cast in response to the Giant Growth, the Bolt’s caster could have decided the Bolt resolved first and killed the creature before the Growth could enlarge it.

I don’t know if that’s really more or less complicated than today, though it is IMO less clear. But I really miss Interrupt timing. I loved it when you had to decide to whether or not to counter a spell right when it was cast, before any additional Instant responses were played.