Misstep is a card I've gone back and forth on. I really don't know where I stand.
On the one hand ...
I've never felt like I've heard a compelling gameplay argument against Misstep. I hear people say Misstep is suppressing cards all the time that I really can't wrap my head around. There really aren't many one-cost spells that were performing well in vintage right before Misstep was printed, and stopped being good afterwards. I think there's a chance Spell Pierce qualifies, but that's not a card I hear people upset about not being able to run.
In contrast I can think of lots of one drop spells that are better now than they used to be (Swords to Plowshares, Lightning Bolt), some that are about the same (Pyroblast), and a tons of great cards that were printed after Mental Misstep and have always been vintage playable (Grafdigger's Cage, Gitaxian Probe, Delver of Secrets. Fragmentize, Deathrite Shaman).
However, the cards I hear people mention over and over again, like Goblin Welder and Crop Rotation and Entomb were fully legal and objectively bad (in terms of tournament results) before Misstep was printed. This is not to say that Misstep isn't good against those cards - this is to say ... If you these cards weren't good in 2011, why is "they're bad in 2018" a compelling argument now? What's stopping people from playing them?
Some one-drop-centric analysis feels completely backwards to me. I think one-drop tutors are slightly BETTER against Mental Misstep than they were against Mana Drain - getting your tutor target countered is a blowout, getting your Vampiric Tutor Misstepped is a 1 for 1 with a one-mana tempo loss at worst. Largely I think Storm Combo is a better deck in a field of Missteps than it was before. Storm Combo decks are loaded with one-drops but they also have the ability to adapt to specialized counters more cheaply than control decks can afford to swap them out. The amount of effort a Storm combo deck has to go through to beat a 2010-era control deck with Duress and Mana Drain is pretty high compared to today. Have you ever cast a Defense Grid against a deck running 4 Mental Misstep, 3 Flusterstorm, 3 Pyroblast? That's an easy button the deck has today that it never had back in 2011.
Anecdotally, one drops are better in vintage now than they were in 2011. I didn't do any in-depth analysis (which would be incredibly hard if not impossible to do), but picking random lists from vintage history and random lists from vintage today, there's just more one drops being played now. This isn't evidence that Misstep makes one drops better, but it's evidence that it's not making them so much worse that deckbuilding is distorted.
I particularly like what Mental Misstep has done to blue mirrors in regards to the interplay around Ancestral Recall. The presence of Misstep has turned tons of lucky random blowout games into decision-intense interplay... Does she have it? Can I risk playing Recall immediately? Can I risk NOT playing Recall immediately? Should I bait with Preordain? If I bait with Preordain is she the kind of player that will always burn a Misstep on any one drop, or is she trapping me? Should I wait until I have protection? How long can I wait until my opponent draws more protection than me .... Wait, is that a Preordain on the stack for my opponent? Does she have Ancestral, too? or is she digging for lands? Back in 2010 this game was "Island pass, Ancestral on upkeep. GG?"
Now I don't for a second think that my metagame predictivity is infallible, but it's really hard for me to think of a scenario where Misstep gets restricted and the desired outcomes happen. I don't see how it could make Shops decks worse or Aggro decks better (If you're not running Misstep in your aggro decks, why not? You care about opposing one drops more than your opponent does, and the tempo swing is more important for you). I don't see Crop Rotation taking Academy from being a 30% deck to being a 60% deck ... Maybe it takes it from 30% to 35% ... but was 32.5% really your cutoff?
On the other hand ...
Nothing I said above really matters. Game design isn't about metagame balance and it's not objective. I can think of ways in which restricted Misstep would make the game less enjoyable for me personally, but not much worse. And while I don't think 1 Misstep would have the effect people are looking for, I could be wrong, and I don't think the outcome would be terrible either way.
The fact is some people just aren't having fun right now. I don't really know how many people, or which people, but it's obviously more than a handful.
And whether or not they're right about what would happen post-Misstep, people not having fun is a problem. If enough people end up in that camp, something needs to be done. I don't know whether a Misstep restriction is the right thing to be done (I would worry for the format if we tried to restrict cards until Crop Rotation was good), but something would need to change, and I don't know that I have any other idea that would actually solve the problem of those players not having fun.