Some of you young blokes have posted some fine criteria in here. There is a couple of vital, life long attributes that are missing however.
1. Have a sense of humour. Don't be afraid to laugh at yourself. If you make a meal of it, forget it and get on with the next mistake. There's plenty to be made.
2. Don't take this job or yourselves too seriously. An anal retentive highly strung "professional" is the last thing we want in a multi crew flight deck. You can be laid back and very professional at the same time. NEVER let anyone tell you that you can not.
3. Modern training regimes are far removed from some of the regressive thinking of days gone by. E.G if you were a tie to sim are you more professional than they guy that does not? Of course not. If you have a tattoo or an earing are you less professional? Of course not again. I do not have either but don't begrudge anyone who does as they make NO difference to someones flying ability or dispatch of their duty.
4. It's a great industry, don't ruin it by walking around with a pole up your rectum!
Good luck with your careers.