I can't think of a single moment, but I'm sure it was during that very first two-week trial (back when they had two-week trials, before they shortened it to one-week with Going Rogue, and then got rid of them altogether for Freedom) when I created my first "mains".
I've said this before, I'm sure, but I've loved superheroes since I was a kid. I grew up, not only on Superfriends, but on every weekday afternoon and Saturday morning superhero show there was. Plastic Man, the Drak Pack, live-action and animated Shazam, Electra Woman and Wonder Girl, the Incredible Hulk, Wonder Woman, Mighy Man, the Mighty Hercules, Mighty Mouse, the Mighty Heroes (... awful lot of "mighties" in there), the list goes on and on.
So, basically...
The first time I hit the character creation screen, my thoughts were pretty much "eh, whatever, might as well try this game", and created a fairly random character. The second time I hit the character creation screen, my thoughts were pretty much "the entire universe is at my fingertips."
I cracked my knuckles and never looked back.