I'll kick things off with my view on Mentor. I would like to see Mentor itself restricted. I see Monastery Mentor as the most problematic card in the deck, and it's far too powerful for an unrestricted card. The problem with Mentor is that it's a moderately cheap threat that simultaneously destroys the opponent very quickly when it gets going, while also generating card advantage. The main kicker is that it's so incredibly difficult to answer the card efficiently.
Vintage is a format full of broken cards, but thanks to Magic's wide card pool, there are efficient ways to answer the most broken finishers. Historic dominant cards such as Tinker and Yawgmoth's Will will win a game, but they each have answers that can completely and utterly shut them down. Tinker for Blightsteel Colossus getting Plowed is a hugely punishing play for the Tinker player, as he lost two mana and -1 card in the exchange. These powerful cards require finesse to use correctly because you will be heavily punished if things don't quite work out.
On the other hand, Monastery Mentor has almost no efficient answers whatsoever. First of all, we're in a world where the most commonly played Counters aside from Force of Will are Mental Misstep and Flusterstorm, both of which do not hit Mentor. Single target removal such as Plow will usually leave behind 1 or more Monk tokens, which means that Mentor is a game-ending threat that beats single target removal. Sweepers are required to clean up the Mentor completely, but are only capable of 1-for-1 against Mentor, in general, no matter how many Monks are cleaned up.
Sweepers inherently cost a lot of mana. The cheapest sweeper that can guarantee an answer to Mentor is Toxic Deluge, but it costs as much as the Mentor itself. Let's consider what this means: a Mentor player can casually throw out his game-ending threat into an opponent who has the 'perfect answer', and he doesn't get punished whatsoever. Both players remain at card parity and mana parity. This is in stark contrast to a card like Tinker that requires careful maneuvering to finish a game while avoiding heavy, heavy punishment for failure. Balance is the only card that I can think of that really well and truly destroys the mentor AND generates mana advantage, but it's restricted, and also a lot more conditional in that it can be very poor in many board states.
Cards like Dread of Night are proactive ways to slow down a Mentor, but using them is incredibly risky, as they don't do anything by themselves. Keep in mind that a Mentor deck is not one-dimensional. Mentor isn't the only card in the deck. They still have the Gush engine and Blue cards. You're playing an enchantment that doesn't do anything against a player who can easily outdraw you and win through attrition.
Given how powerful Mentor is as a game-finisher and the lack of real answers to it, I believe that it is far too powerful for an unrestricted card, and the dominance of Gush Mentor in the Lodestone restricted world proves to me that restriction is probably a necessary action. I understand that there are many people who generally disapprove of restricting creatures because "come on, it's Vintage, creatures are a joke in Vintage," but if you believe that, then I would encourage you to take a second look at the world we live in. Creatures aren't the joke that they used to be.