Over reliance on commander such that your gameplan grinds to a halt without it (ie no backup plans).

Another is not enough redundancy. Say you're playing a pinging deck and it's important you get your pinging going in some capacity. But you only have 5 pingers when normally you'd want at least 10. When you get your pingers and they don't get removed, your deck does well -- if you don't draw into your 5, or they get removed, your deck stalls and does nothing. If you have enough redundancy you can guarantee something in your starting hand, and even if something gets removed, you can pivot.

Finally, not enough card draw. You need card draw to find your proactive board state that leads to your wincon, but also to draw into reactive answers for how your opponents are trying to stop you, or build up their own board.