Multitrack, period, changed my life.
Yeah. In high school, a mutual acquaintance loaned to my friend and me a four-track that his uncle wasn't using (I think it was a Tascam, but I can't remember). That changed it all for both of us; suddenly it didn't matter that our "band" had only two people in it. We made endless recordings on that, a Marantz four-track that we eventually bought, a Tascam 388 (we felt like total badass pros with eight
channels of reel-to-reel
tape; I even asked my family for Ampex tape for Christmas one year), and eventually several members of the Cakewalk series of multi-tracking software.
He went on to get an audio engineering degree; I studied Math and wrote my thesis on digital audio compression. I don't think either of us would still be into music if it weren't for fortuitously stumbling across recording equipment when we were still young and had copious amounts of free time.