Simmons.
He misses layups and dunks more than he should, and to me, doesn't go strong to the basket even though he drives. He's just not a consistent offensive threat, though he occasionally has the dominant offensive game against a peon team. I just think this iteration of the team is in big trouble, the Celtics provided the blueprint a few years ago with how to defend Simmons, and the playoff teams tend to have the talent to execute it. That task will probably be even easier this year with the spacing issues of Simmons / Horford / Embiid. Last year was probably their best shot to win it all.
He's not the only problem though, still amazing Embiid looks so out of shape. Different year, same questions, yet Brown seems to just be absolved and not held accountable -- though I guess lack of accountability isn't limited to that as we see plenty of that on the court.
If you're bullish and positive about them, more power to you, wish I could share your optimism. I just see a really good team talent-wise, that is fatally flawed to a point where I don't think they can beat MIL, LAC, or LAL minus massive injury problems for those teams, and may very well struggle with some of the solid but unspectacular teams in the East (IND w/ Oladipo, TOR).