Interesting discussion this.
I have seen and spent time with a very diverse group of traders. From very successful independent & prop traders, traders at firms, traders at banks, average traders, losing traders, losing traders who think they are good traders, etc. The three things that really stand out separating the traders comes down to: 1) Discipline 2) Conviction 3) Guts.
In my experience, having an edge to pull an income from the markets is actually not that hard at all. I would go so far as to say it is easy. Some of the most consistent traders I know have particular setups, and they just don't really question it. They don't make a killing, they just grind it out, working their small edge.
Mentality is too general a word. The more specific problem: The majority of people have no discipline. It takes a huge amount of discipline to know what your specific edge is, sit infront of a screen and only take those setups. To only trade your edge, entires & exits.
I do NOT think the problem is exactly about having a profitable strategy. It's about having a profitable strategy, and trading that and only that.
The average person simply can't sit infront of a screen all day, every day, to only take one very specific setup. Even if it were to make them more than their current income.
If you can't follow an exercise plan, can't follow a diet, can't follow a study plan, etc - It is unlikely you will succeed at trading until you can address those issues.
This is one of the key reasons why there is a correlation between successful athletes following on to become successful traders - it is the discipline aspect.
Subsequently, it is also a key factor in why there is very little correlation between being successful in a white-collar job, to becoming a successful trader. Most 'real jobs' (as I call them :-) ) do not require and test your discipline on a daily basis.