I agree with Skier, in that you don't have to be advanced to have fun skiing or to "get down" "expert terrain".
Also from a visual perspective, when I see someone skiing with little to no separation and twisting their skis to a steering angle using momentum from their upper body to get the skis turning, instead of tipping and having the legs turn/move beneath them, I naturally place them at a level below advanced. If I know this person, I have a strong desire to get them to ski "better". However, at least one person I know who skis like this is quite happy and can get down any runs in resorts in eastern Canada without difficulties, and doesn't seem to be interested in learning how to carve clean turns and acquire a level of skiing with good separation. I know, right? How can someone not want to ski faster with higher g-forces and better control? Different strokes for different folks
.
Also, looking back at my own development, I very quickly learned to tip my skis and bend my skis early on, but separation took much longer to learn. I was definitely not an expert, but was skiing all eastern runs (for reference anything on the trail map at Tremblant and Jay Peak) at high speeds with ease, despite my lack of angulation and counter rotation, and only a little better than that when skiing narrow steep chutes/cliffs with lots of mandatory air and half a dozen danger/death/no trespassing signs to dissuade people from going anywhere near them. I had developed my poor form to a high degree, compensating for poor form and poor technique with high balance skill, high strength, and quick reaction time.
However I disagree in the reason for learning to ski better. It's not to look better for the chair lift audience; it's because the better you get the more fun it is.
As to the powder/ice/crud/bump requirements for "expert" ; it's just a matter of what one is used to. If someone never saw powder, they could still be an "expert", despite still needing a few more days of learning to transition to being able to do a decent job skiing powder. I would consider anyone seriously into racing and with some experience racing an expert, despite the fact that many skiers could ski bumps better than they could. I expect I would still consider anyone who has been skiing 100 days per year for a few years in bumps an expert, even if they couldn't make it down a race course in half (EDIT: I MEANT TO SAY LESS THAN TWICE) the time it would take the average racer.