What was Hill's defense like? At the general level, you kind of need to compare Flagg to comparably-sized forwards that do everything well: score, pass, rebound, and defend. That's where the Pippen and Tatum comparisons make sense. Once you go a little deeper, you can begin to identify the areas where those comparisons are lacking.
Pippen doesn't have Flagg's strength and he doesn't truck through guys and seek contact like that. Tatum isn't the athlete that Flagg is; he doesn't cover the court and get off the ground and in the air like that.
The initial popular comparisons were upgraded versions of Kirilenko and Marion with on-ball upside. Those aren't bad, but does it make more sense to start with an actual on-ball primary creator and then work some modifications from there? I don't know; either way works.
Jim Boeheim said this the other day:
"I think he's almost — to me he's Bird-like a little bit," Boeheim said.
...
"Except, he's quicker, faster, more athletic; he doesn't shoot (the ball) as well, but his shot is coming," Boeheim continued.
"His shot is there; it may not be there this year, but it's there. He's got a good release.
"The thing that he has — and I've talked to Coach K (former Duke head coach Mike Krzyzewski) about this — he has an edge to him," Boeheim said.
"He's not backing down from nobody."
Someone mentioned Jimmy Butler in this thread; I think it was Ice Man. I've really warmed to that comparison. They do a lot of similar things, including their mentality/approach to the game. Simple but effective playmaking, bully-ball drives, abusing mismatches in the post, active cutting and offensive rebounding (including own misses), screen setting, a knack for jumping passing lanes, etc.
The biggest differences I see are that Cooper is about 2 inches taller, has 5-7 extra inches of standing reach and wingspan, way longer strides/legs, and a higher center of gravity. This extra size allows him to be more of a vertical and horizontal presence on both ends rebounding, blocking & deterring shots, rotating and providing help on defense, finishing plays (oops, rolls, and cuts), and getting out in transition.
Skill-wise, Jimmy's handle is way tighter, and his slashing game is more creative with his footwork and fakes. Coop needs to work on his one-foot finishes.
Flagg looks like he'll be the superior three-point guy hands down but I'm not yet sure about the mid-range. Butler is a pretty good SMR (short mid-range) shooter.