I think more mistakes are made by teams in Summer League than any other time of the year. Every team wants to love their own prospects, because it makes them look smart. So when teams get to summer league and one of the players they drafted or found as a free agent, plays well they see it as validation of their brilliance and foolishly sign the player to contracts or plan on that player in the upcoming season. It is guaranteed to happen. It is just about guaranteed to fail.
The reality is the summer league is a big man absent league that includes rookies and the 9th to 14th man on NBA rosters at best. In other words, rarely is there more than one player on the floor that is going to make a huge contribution in the upcoming season.
Remember last year Steve Novack of the Rockets was a summer league star and never saw the floor in the season.
Here is what you should look for.
1) The top players should be dominant. For Durant and other high picks their should be no question when you watch the game that they are the best player on the floor. How they shoot and what numbers they put up is not as important that they are obviously the best players on the floor.
2) Look for rebounders. Rebounding is the combination of a skill, athleticism and desire. Some guys run out of the skill or the athleticism and they can't rebound at the next level. Others don't have the desire and when it gets more difficult they fall off on the boards.
3) Keep an eye on shot attempts. If a player who should be shooting isn't it might be a red flag that they are struggling to get their shot off when the game speeds up.
Finally -- DO NOT FORGET --- these are at best the 9th man on a team. There is not a veteran anywhere to play a veteran trick or lay a big hit or bury a kid on a pick to see how he reacts. DO NOT OVERPLAY SUMMER LEAGUE. Keep an eye out some team will make a mistake because of Summer League.