The Commander Rules Committee announced in their quarterly update that the following cards were banned in Commander, effective immediately:
You can read their justifications in the associated blog post. They also published a follow up FAQ with some additional clarifications.
Banning Nadu was slightly surprising to me: it's a busted card, don't get me wrong, but there are a lot of busted cards in Commander. I have a fairly strong Mizzix deck which frequently wins by taking a single long turn, drawing through the deck and generating mana until it sets up a killing spell, but it can miss. There are only so many win conditions in the deck, so it's possible to draw the wrong cards and not win the game even after casting a bunch of spells. Hey wait a minute, this sounds pretty similar to Nadu. What's going on here?
I'm not going to fully compare Mizzix to Nadu, but the fundamental difference is that Mizzix reduces only spell costs and requires you to figure out everything else. Nadu gives you cards and the lands to cast those cards directly, and does so efficiently.
I think another major factor in Nadu's banning was its unique popularity. It took over Modern almost immediately after its release and avoided a Modern ban for longer than expected due to some scheduling quirks*. Lots of people got to know Nadu as a strong card, and it stayed relatively affordable, so it made its way to Commander tables and started taking over games. So, despite my initial surprise, the ban makes sense to me.
* The dates for potential ban announcements are set in advance, and the ban announcement date after Modern Horizons 3 happened to be the same week as a major Magic tournament. Wizards decided to announce in advance that there would be no bans on that date, to enable people to properly prepare decks for the tournament.
Mana Crypt, Jeweled Lotus, and Dockside Extortionist all check three boxes:
I don't own any of these cards, mostly because they're really expensive (point 3), but also because of their incredible isolated power level (points 1 and 2), leading to the occasional bad social experience at my tables. It doesn't help that these cards are nearly themeless; I love including slightly weaker cards that fit the theme of my deck, and these cards will never do that.
* At least, among cards that aren't on the Reserved List or limited in supply for some other reason.
It takes a lot to get banned in Commander, a social format that still isn't all about winning. In fact, these are the first bans in Commander in three years. The banned list is reasonably short given that over 27,000 cards are currently legal in the format. I don't disagree with any of the cards on the banned list—in fact I'd ban even more cards if I ran the world, but that's a topic for another time.