It started with Amazing Spider-Man # 238 (first appearance of the Hobgoblin). I had a vague recollection that the Green Goblin had ruined Spider-man's life and killed his girlfriend. When it seemed like a new version of the Goblin was starting up and I was on the ground floor, I was hooked. Over in PPTSS, Doc Ock was at war with the Owl, with Spidey and the Black Cat caught in the middle.
An Amazing Spider-man Annual introduced me to Captain Marvel (Monica), which brought me over to the Avengers, the Vision and Scarlet Witch mini, and an Annihilus-based crossover with the Fantastic Four (hence my first exposure to Byrne's awesome FF run). Avengers Annual #10 made me fall in love with Rogue, and since it seemed she was just joining the X-Men, I started reading X-Men, New Mutants, etc. I did some back-collecting of the X-Men (all the way back to Giant-Size #1).
So at that point I was fully involved in the worlds of Spider-Man, the Avengers, the FF, the X-Men, the New Mutants, Alpha Flight... even some occassional Iron Man/Thor/Cap issues (though they never sucked me in like the team titles or Spidey). Occassionally I would pick up MTU or MTIO, or What If?. Dabbled in some Batman, though other than Batman and his villains, I found DC boring. And all this collecting lasted straight through until it all got crappy in the 90's. Money was tight and my enjoyment of comics was low, so I took a break for about 10 years.
They sucked me back in with JLA/Avengers, but now my pull list is waning again. Maybe I only have a ten-year tolerance for comics.