I've often heard it said that the Urza Block (Urza's Saga, Urza's Journey, Urza's Destiny) was the most powerful, broken block in Magic history, with decks able to win on turn 1 or 2. I also saw it stated (in some MaRo article) that something like 20 cards were eventually banned from that standard format in an attempt to restore balance. However, I've never seen a good, detailed explanation of what, exactly, went wrong, which cards were to blame, and how the uber-powerful decks of that era actually worked. Can anybody here offer an explanation or a link to an external source?
Preface: I've been drafting this for a few days now. I focused on the cards that were banned, because those were the most broken. I'm sure there were dozens of other combos not included here, and this is by no means giving the fullest extent of how the cards could be used to maximize their broken-ness.
The first step is to take a look at what cards were available at the time. Mirage had just rotated out, which leaves us with the Tempest block and of course Urza's block. This leaves us with some very powerful 0 and 1 mana artifacts: Lotus Petal, Mox Diamond, Ornithopter, and to a lesser extent, Spellbook.
Note that with Lotus Petal and Mox Diamond, you have yourself 2 mana available on your first turn without playing a land. (To be fair, both of those are part of Mirage, not Urza's.)Those mana can go into a Grim Monolith, which in turn gives you 3 mana when tapped. Make your land drop a Tolarian Academy and suddenly you have 6 mana available on your first turn, and you only needed four cards to do it. I think we can agree that 6 mana on the first turn is broken in an innumerable number of ways. This led the Academy to be the first to be banned.
Along with the Academy was Windfall. This was broken for a couple of reasons. First, consider a card like Divination, which has a card advantage of 1 for 3 mana at sorcery speed. (You draw two cards, but you had to spend 1 card to do it). When you also consider Azure Mage it becomes clear that in blue, 3 or 4 mana is appropriate for 1 in-hand card advantage. Now, consider how fast you blow through cards here: you dropped four cards (lotus, mox, grim, and academy) as well as any other 0 drop artifacts, and use a Windfall. You discard your 0 or 1 non-playable cards and pick up 7 more. Your opponent dumps all 7 of his and draws 7 more. You just gained 6 or 7 card advantage for 3 mana. That just isn't competitive.
The other aspect of Windfall is that your opponent is also playing combo. It was not uncommon for people to mull down to 4 to get their combo. While this will lower the card advantage you get from Windfall, it essentially wastes your opponent's mull! He now has to settle for whatever 4 or 5 crap cards are on the top of his deck. If you went first, then your opponent has not had an opportunity to play, but now is facing you with at least 6 cards on the battlefield, the ability to generate 6 or more mana a turn, and to top it all off, you just gave him a complete crap hand instead of the one he mulled to get. Bummer. So understandably, those were the first two to get banned.
In the next round of bans, you see Dream Halls get the axe. Now, without Academy and Windfall to pump up your early mana, this isn't as effective. It was costed such that you can't use it until turn 5, and you have to use two cards to do it instead of one. So since they were expecting you'd use this on turn 4 or 5, it would end up being very damaging. And under normal circumstances, it is. But there's one small problem. You discard that other card instead of paying the cost. This is supposed to make it so you can't use it. But Yawgmoth's Will made it so you could use it. If you Dream Halled two cards, you could cast the other two cards with Yawgmoth's Will. And with all the mana you have and card draw available, it's reasonable to pull this off.
Another one that was banned was Earthcraft. This card used to look like this:
However, it was decided that splicing text onto cards was a bad thing. So it was changed to
in the name of being easier to work with. Unfortunately, it had the side effect of making creatures with summoning sickness able to tap for mana, which is decidedly overpowered.
Fluctuator was banned because there's about 40ish legal cards with cycling, and they're all (Cycling 2). So they all basically let you loot them for free to burn through your deck. Cycling for free is broken. This combo didn't take much to come together.
Lotus Petal was banned for the combo listed out above, and I'm sure there were dozens of variants of it.
Recurring Nightmare was banned because three mana and a creature (probably 0 mana) was too little to return a creature from the graveyard to the battlefield, considering it goes back to your hand, not the graveyard (even if Yawgmoth's Will is active). It's even worse when the cards coming back are Great Whale, which yields infinite mana (although the infinite mana combo requires four lands on the board, and most games didn't last even that long!)
Time Sprial was the last of the second round to be banned from standard. It had the same problem windfall did, where you got to draw seven cards (including spells you've already played! How convenient!) and get the massive card advantage. Now this is 6 mana, not 3, and we know there's a gigantic difference between 3 and 6 (way way way more than twice as much). But as we've established, you can get 6 mana on your first turn... And to top it all off, you get to untap 6 lands. Umm, yes that's just a little too powerful.
The next big card to get the axe is Mind over Matter. This is simple: you discard (which we've established there's already massive card advantage here, which lessens the penalty...) and you get to untap an artifact (with academy, mana, or the mana artifacts themselves), creature (with earthcraft, mana, or Birds of Paradise), or land (mana). So you get even bigger mana amounts.
Memory Jar was so broken it was the only card to get an emergency ban. Basically you get to draw and play with 7 cards. As we've established your hand empties faster than a AK with a 14 round clip, you're going to get most of those cards out the turn you play it. And since 5 mana isn't all that much to get on turn 1, you might end up with this on turn 1, which when combined with another draw combo will give you access to 20 or more cards on turn 1. Broken? Yep. Unlike windfall, this doesn't kopper your opponents starting hand, as he'll get it back at the end of the turn. But you'll have another 4 or 5 permanents on the board, so what's he really going to do to you at this point anyway?
Like I said, these probably have even better ways to get the most out of them. But hopefully this gives an idea of exactly how these things were broken, and why Wizards reacted the way they did with all the bans. For what it's worth, this is the first genuine test of Wizards reacting to the fan base, and I would go so far as to say their biggest mistake up until that point and even to this day (needless to say they learned many lessons from it!)
There have been several times in MtG history that the game was broken for a while. Black Summer, with the proliferation of Necropotence decks, Affinity decks that abused cheap artifacts and artifact lands, and Combo Winter that resulted in the largest number of bannings. Mark Rosewater discusses why cards get banned and restricted. While not all the cards are from Urza block, about half are, and Mark includes some interesting information about the prototype versions of some of the cards and why they are broken.
A timeline of DCI banning/restrictions is available if you just want to know the timing of the various bannings. This article has some nice background on Combo Winter, some examples of Pro Tour decks, and links to tournament reports of what decks were being played at the time. It has an interesting piece from Randy Buehler on why Memory Jar was emergency banned, the first and only card to be banned before its release. If memory serves, Wizards even offered replacement packs if you opened a Memory Jar, because it was unplayable.
I am certainly no guru, and I play MTG very casually compared to some. I think the reason the Urza block got mixed reactions was the introduction of some powerfully good cards, and the number of these cards, that these days (certainly in modern format that my group plays mostly) probably aren't that powerful at all! The game of MTG has changed over the years.
My group have fond memories of that block! Gaea's Cradle. Serra Avatar. Defence of the Heart. Congregate. Crazy days!