From: Mozzezz | #037
Wow really, I had a lot of fun with Urza's, I'm not sure if I'd call it the best set, but I don't think it was so bad that they had to yell at the design team.
It's not that it was bad it's that competitively it was a balancing disaster. Urzas had more bans, erratas, and overall issues than any other set in the game, including a few of the most overpowered cards in the game.
Tolarian Academy completely broke the game and is widely considered the strongest card ever made, especially in a set full of cards that could completely abuse it.
Yawgmoth's Will allowed players to utilize mana-earning spells and low-or-nothing cost spells to great advantage, being so abusable that it is banned in every format but Vintage, where one designer even suggested it should be the only card to ever be banned in Vintage.
The blue free spells were incredibly powerful, especially given multiple mana producing lands that were abundant(a 7 cost free spell would let you untap 7 lands) and they had to issue an errata to change the effect so that it wouldn't happen when you didn't play the card(like returning a creature from the graveyard to play).
Morphling is widely considered the most powerful creature ever created, partly because it's incredibly reslient, nigh impossible to kill, and can provide a killing blow for control decks.
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Fresh from my first justified ban. Ever!