1. Velocity: live arm w/movement(arm-side, glove-side, sink, or cut) or a mid-90's heater.
1A. Command: you can have a god given arm, but if you can't command the strike zone consistently, it doesn't matter!
1B. Secondary Pitch: Good hitters will hit the FB, you MUST have good secondary pitches in your arsenal.
2. Good Mechanics: will help #1, #1A, and #1B stay consistent, and hopefully keep pressure off the arm to maintain health.
3. Mental Make Up: Heart, Determination, ability to compete 1 pitch, 1 batter, 1 out, 1 inning, and 1 game at a time!
4. Conditioning: Arm, Legs, and Core Strength...and even Mental Conditioning via books, tapes, mentors/coaches.
This is what I've preached to my son for years as his dad/pitching coach. If you have the above traits, IMO you'll get people out and be successful the majority of the time...even on days that you don't have your best stuff.