What's Really Most important when Deciding a career that fits?

This varies from Fresher to Experienced, and yet again it varies between little experienced, moderately experienced or highly experienced (read this in terms of tenure).

First and foremost is realizing what the individual is passionate about and explore opportunities accordingly.  Every position and industry has the career path defined distinctly, one has to match their ambitions and selective of their career.

Once decided, wear the right attitude for the job, there is no single rule of attitude matches for all roles, one should wear different hats depending on the role and circumstances.