With the exception of a few elite teams like the Miami Heat, Oklahoma City Thunder and Los Angeles Lakers, every NBA team needs a player to elevate his game and push the squad to the next level. Even those elite contenders still wouldn't mind having a player like that.
These are the x-factors, the guys who have the most volatility when it comes to impacting the team. By definition, the x-factor isn't the one who emerges from nowhere and becomes a key contributor, but rather the one with the biggest impact on the team's success.
If that player breaks out, the team is going to make a leap into the next tier of NBA squads.
As a note, though, rookies will not be eligible for these rankings, which rank all 30 x-factors from worst to best in terms of overall performance. First-year players are too unpredictable to be true x-factors, as we have no way of truly gauging their professional chops quite yet.