So you've seen by now that Stoneforge Mystic and Jace, the Mind Sculptor have been banned in Standard. (If you haven't, I suggest reading the post below this one, and pay very careful attention to the 1 circumstance where Stoneforge Mystic is legal to play).
What you may not have read is why they banned these two, along with why the War of Attrition Event Deck decklist was allowed to stay Standard legal (the one exception to Stoneforge Mystic's ban).
Stoneforge Mystic was no mistake..at least, not initially. In Worldwake and Zendikar block in general, Equipment played a very small part of the set that Stoneforge Mystic was just a nice Equipment tutor. And, they figured, it'd help the Equipment subtheme in Scars of Mirrodin block.
The problems started in Mirrodin Beseiged. The design and FFL (Future Future League testing) teams didn't realize the impact of Sword of Feast and Famine to be as strong as it ended up being. However, Wizards of the Coast decided to let it go for now and see how people would react to the power level in Standard. Then New Phyrexia came and screwed things up majorly.
The design teams for Scars of Mirrodin block knew they wanted Living Weapon to play an integral part in the Equipments of the block starting in Mirrodin Beseiged. They tried to make stuff like Lashwrithe or Bonehoard that were great to search for but weren't necessarily at their best when cheated into play early from Stoneforge Mystic's second ability.
Unfortunately, they printed Batterskull after a chance in direction from the guys who made the cards. Instead of its last effect being as printed (3: Bounce Batterskull, essentially), they had it as "8: Put a 0/0 Germ token onto the battlefield with Batterskull attached." The bounce effect, while being powerful, has proven too powerful: For 5 total mana, you can block a creature, let damage go through, lose your Germ, then pay 3 to bounce it back to your hand and 2 to replay it thanks to Stoneforge Mystic's second ability.
As the format got more and more Stoneforge Mystic-based, WotC decided that, even with a few months left in Standard, damage was already being done (drops in attendance, general player hate for Mystic-based decks) and a ban was warranted.
Jace, the Mind Sculptor
Jace ended up being a design and development AND R&D AND testing mistake all the way around the table. As they were getting Zendikar off the ground, they realized its Planeswalkers may be too weak in a general sense (and really, aside from Nissa in Elf decks have you seen the Zendikar set's 'walkers anywhere in competitive Magic?).
WotC decided to take a big swing at how Planeswalkers were designed and they needed a big draw for Worldwake, so the basic ideas of Jace, the Mind Sculptor (4-ability Planeswalker, new, super powerful) were born.
At the same time, WotC was trying to give Blue some of its power back. Back in Lorwyn block, Faeries was the dominant deck in the metagame. Most of those Faeries were Blue, leading to a lot of U/x decks (Decks primarily with Blue and some other color). It didn't help that Cryptic command was in the same set as those Faeries, giving Blue a lot of god weapons for a Faerie-based deck.
WotC intentionally cut Blue's power level in Shards of Alara block and Magic 2010. Blue had fallen off, so with Zendikar block they wanted to give Blue some of its power back.
They were going to make its abilities these instead (and I'm shorthanding the abilities):
+2: Mill the top 2 cards of target player's library.
-8: (the final ability that ended up on the printed card)
Thought to be too powerful, the ultimate ability was raised to -12.
UNFORTUNATELY, they also felt milling 2 cards was too powerful for a +2 ability, so they changed it to "+2: Scry 1 or Fateseal 1" (again, not the exact templating but a shorthand using known mechanics that do the same thing). They didn't realize how powerful it was to keep players off their most needed cards on a regular basis. (Fateseal on a spell is one thing; Fateseal repeatable from a permanent is dangerous)
Wizards of the Coast tried to print answers. Phyrexian Revoker, Despise, Hex Parasite. Unfortunately, the answers were too low on power and didn't matter. Jace, the Mind Sculptor was still Planeswalker #1, and its play amount in Legacy and Vintage has helped cement its popularity and power level.
In their defense, it was only the 14th different Planeswalker to exist and the 2nd to be the headline Planeswalker of a set.
But now they'll be judged based on their admitted mistakes. They no longer have an excuse to make a super-powerful Planeswalker. And now they have no excuse for making Tinker-like effects on cheap creatures.
Why Is Stoneforge Mystic Banned Except for 1 VERY SPECIFIC Deck?
This is fairly easy. Wizards of the Coast had Stoneforge Mystic penned for the War of Attrition deck while it was still a relatively meaningless $6 card. They had no idea at the time how powerful it'd be. Seeing as the Event Decks are meant to be FNM-legal, the best way to satiate both concerns (it's too powerful, but it's in a product intended for use at FNM), they allowed the exact War of Attrition decklist to be legal so players can play War of Attrition straight out of the box.
Hopefully that goes a ways to explaining the reasons for banning. What are your opinions? Did WotC need to do anything? Could they have done anything differently?