A lot of people in my family measure success by the amount of money you make and this seems to be the way for most people that I run across. If you have a six figure income, you are very successful. Doctors tend to be at the top of the list and get that wow factor from anyone you talk to though, not just people in my family.
The whole "life and death in their hands" aspect of being a doctor might make up a little more of the way they are viewed than just the money they make, I would think.
I do agree about the six-figure income being a measure of how success is viewed.
If you make six figures, ANYWHERE in the US, you are doing well for yourself. Now obviously, $100k goes a lot farther in some places than others, but you should be living comfortably anywhere if you are making that much or more.
Now.....whether YOU or others consider that "successful" is a personal measure. If you thought you'd be a multi-millionaire by 35 and in reality you're "only" making $250k, you might consider yourself to not be a failure, but maybe not "successful".
Success is all about perspective.
Here's a sports-related example:
College football
A coach that wins 6-7 games and goes to a minor bowl game is considered "successful" at a school like Boston College, Minnesota, Wake Forest, Duke, or any smaller conference school. Do that for 5-10 years and the fans love you.
Now, win 6-7-8 games at Texas or Alabama? You are fired, even though are playing a tougher schedule than the other teams I mentioned, and none of them could have a winning season against your schedule.
Why? Because of expectations, and the definition of "success" by different people/schools/organizations/etc. "Success" at those schools is winning your conference, playing for the national title and going to a major bowl game if you are not playing for the title.