I would define expert in the reptile hobby, or whateve ryou want to call it, as someone who just knows a lot..of course you can never know everything, but imo i would listen to some people on these forums over others any day just because i know some of the animals they've worked with and can tell how intelligent usually by some of the posts they make.
I dont think their is a defined rank of expertise because this can never be made, but one could say that this person really knows their stuff. I think its all about experience, more then what you keep showing your expertise. Because someone can own a burm or retic, and most would say you need to know a lot or whatever to keep those, but they know jack off and say they know everything. Or someone who owns a 100 snakes is a better example, doesnt meen their smarter then someone who owns 1..get my drift?
thats just my 0.02