In order to answer this question, we need to define 'great.' Many attributes can be associated with teams that are considered great. Brazil has 'flair' and 'rhythm,' Germany is 'disciplined' and 'measuring,' Barcelona is 'technical' and 'quick.' But what is the one trait they all have in common? The answer is - THEY WIN! To be considered great you must win, and you must win often and over time.
What does it take to win? The simple answer is GOALS. Back to the original question, what makes a team great? In my opinion, A GOALSCORER. You can tell me all you want about passion, discipline, technique. A goalscorer solves all those problems. It gives teams belief that they can win any game at any time. Defenders are more focused and play a little bit harder because they know if they stop the opponent, their goalscorer only needs a half chance. Do not confuse a goalscorer with a player that scores a lot of goals, especially in the youth game.
Germany has Mueller, Bayern has Lewandowski, Leceister City (having and great season) have Vardy and Marhez, and Barcelona have Messi, Neymar, and Suarez. When has the U.S. been their best? When McBride and Dempsey were scoring. In a game that is so team oriented, a single player - the goalscorer - can change a season or more.
Why is it so difficult to develop goalscorers? I am sure many people will disagree with me, but goalscorers are not born. They are not natural. Goalscorers are created. Like anything made, you must first have quality raw materials.
So often in the youth game, the goalscorer is the best athlete. We (coaches) play the biggest and fastest as forwards because they run by defenders and shoot (often times really hard). Some coaches even consider technique. Their forwards might not be the #1 athlete, but they are good with the ball, keep tight control, and yes they can shoot. These players usually get a lot of chances to score, however miss way more than they make.
I agree that athleticism and technique should be part of the equation. However, goalscorers need to be DEMANDING. This is often described as having an edge. With young players it is also mistaken for being angry. These players sulk or yell at teammates when they get frustrated. They often times cheat in games or practice, or at least use the rules to their advantage.
Goalscorers need to be RISK TAKERS. They will slide tackle a lot. They will often not listen to all the coaches instructions because they are trying to gain an advantage by being first on the field. At young ages, goalscorers often miss instructions or skip steps because they are already thinking about things three steps ahead.
Goalscorers are COURAGEOUS. They may volunteer to do things first, blurt out answers, or they may be the kid willing to say hello to the new player.
And finally, goalscorers are EFFICIENT. Some are minimalists where they do little running except when they are involved. Some may try to skip ahead in activities if they feel they already know a step.
You may be fortunate and have a player (or two) like this on your team. Work with them on technique, play them often as forward, challenge them in training with different rules or restrictions. If they are also athletic or technical make training unfair (at times) by giving them different and tougher rules then the rest of the team. Goalscorers must be able to see space and predict opponents movements. Put them in situations that make their situation more difficult than the game.
If you want a great team, develop a goalscorer or two. Athletes will score at young ages, but a true goalscorer will get goals against better teams, better athletes, and with few chances. Look for those players that are demanding, take risks, have courage, and are efficient.