I *can't* remember the last time I've even been in the abandoned sewers other than to visit the kraken; but I *can* remember when I finally managed to get a toon up to around level 14 and thought it might be useful to map the *entire* sewers out one evening.
I remember AP and GC having multiple instances, and then KR and Steel for a bit as well.
I remember running around as a noob excitedly chatting with my friends as we oooh'd and aaah'd each time we saw a higher lvl toon. And I remember the day when I finally saw my first 40th lvl toon!
I remember deleting a 33rd lvl tanker because of how poorly he was slotted and atm there wasn't any way of being able to fix him
I remember finding the forums for the first time and thinking to myself- "Oh god, this addiction just keeps getting worse!"
I remember the day I officially recognize as when I knew I liked this game- saw this defender wandering the streets in AP (I was a big strong lvl 6 tanker!) acting a bit lost so I took some time to try and help her out. Ended up she was actually 9 years old in real life. Her RL mother and father came over about 2 minutes later and the 4 of us (yes, they had 3 accounts- and in fact, they had 4 so their son could play at the same time as well) ended up running around for together for several hours.

All the graphics and content and such aside- that's what did it for me.
I remember the first time I faked being *sick* so I could stay home from work to play CoH.
I remember the first time I began to think I was addicted.
I remember deleting a 27th lvl toon because he didn't have the isolator badge.
I remember get hit by a sniper for the very first time. I was in the middle of nowhere without a mob in sight and spent a few minutes trying to figure what the the bejeepers was hitting me.
I remember the jealosy and loathing as I watched 14+ toons go leaping/flying off into the sunset while I *painfuly* labored up countless flights of stairs looking for CoT in KR.
I remember the countless times my stone tanker was asked "What the heck are those purple thingys floating around your head?@!"
And I *just* remembered that I need to get to sleep as I need to be up in 4 hours from now!