Summer league is meaningless, so many players looked good and turned out to be complete busts or vice versa, it’s hard to put any stock on the summer league when evaluating players, it’s weird how different they look when they play actual NBA games
there is a lot of stuff to look out for, you can get pretty good idea of young prospects and how their games function. granted, they're not playing top level competition so a grain of salt and all that, but you can generally learn some good stuff about players. Like, Trae struggling was surprising to me because he seems tailored for the SL, and the Summer League usually makes life easier for offensive guards, so maybe the question marks about his athleticism are worth keeping an eye on.
Just because a player does well at SL doesn't mean he'll do well in the Regular season, so you kiiiinda have to figure out what to look out for. Like, if an undersized guard is just sinking off balance shot after shot, maybe there's reason to question how well that will translate. but, say you are looking at a guy like Wendel Carter Jr.
He's moving well laterally on defense, something that was supposedly a problem for him in College, he's showing good instincts for shot blocking and he's generally making the right decisicions and doing the little stuff. That stuff generally translates well and can tell you something about a players headiness, their focus and how "easily" they'll take to NBA level competition.
Again, context is everything and it is absolutely not a direct analogy, but stuff like work ethic, attitude, defensive fundamentals, setting and navigating screens, etc. If you see young players who have that stuff down pat you can generally make an educated guess that they're going to be doing alright in the regular season once they adjust to the pace and schedule.