I'll give my $.02. I'd first like to say that I'm pleased to hear that CharleytheDinosaur's friend had a good time and is eager to try more coasters. When talking about how riders passed through coaster milestones at KI, I progressed through these as such: Beastie (Woodstock Express), then forwards Racer, then Top Gun (Flight Deck)/Adventure Express (this was roughly around the same time), then Son of Beast (with loop) and finally Flight of Fear (Reptar/Flying Ace and backwards Racer were somewhere in there as those were my first invert and my first backwards coaster, respectively)*. By the time I'd conquered Flight of Fear, I thought I could handle anything (the first Vortex ride didn't even seem like a big deal!). Finally, for everyone out there; do NOT force people to ride rides. Showing them coasters and trying to get people to go on them is fun, but forcing someone to go on something they don't want to (or having a possible rider fear that someone will force them on a ride they don't want to go on- trust me, I know!) is never a good idea!
*I wrote the old names as the rides were named this way when I first rode them. Also, Volcano (at Kings Dominion) was my first launched coaster (occurred after Son of Beast) but I didn't include this in the list because it's not at Kings Island.
EDIT: There's one more milestone I left out... Firehawk was my first flying coaster.