Why is Pot of Greed banned?
The simplest terms I can think to put it is:

Your deck requires 40 cards. Imagine you could have a 12 card deck - your 4 best cards, 3 copies each. You'd probably win every game against those 40-card deck losers, since you're always drawing your best cards and deploying them.

Every card you put in your deck makes you less likely to draw those game winning spells. So you replace them with some themed cards to bide your time until you DO draw into them. So it makes sense that a 15 card deck is weaker than a 12, and a 20 weaker than a 15. The important fact is that a 37 card deck is stronger than a 40.

Now imagine Pot of Greed just said "draw 1 card" instead of 2. What it would essentially do is let you get take your 40 card deck down to a 37, because every time you drew Pot of Greed, you would immediately exchange it for the next card.

So if it were draw 1 card, it would immediately be banned because it turns any deck from 40 into a 37 card deck. And if at draw 1 it's too powerful, then at draw 2 it must be even more powerful.

Another very simple way to put it is this: when it comes to making games fun, the key is to make interesting decisions. Pot of Greed has no interesting decision: you play it, you always benefit. It's not like a burn spell where you choose a monster to burn, or a monster that you choose "should I put this out now? Or wait for him to use up his monster kills..." Pot of Greed doesn't make games more fun. It makes them less fun.