@fsecco Eh, I'm kinda tired of restricting threats. There are a million ways to kill Mentor in vintage, and 90% of the good ones are weak to Misstep.
Well the reason that Mentor isn't good in other formats like Modern is due to the prevalence of 3+ cmc sweepers. Mentor is good in Vintage because it's strong against cmc-1 targeting spells. (Should always get at least monk out of the trade)
I agree that free spells like Probe, Misstep, FoW, and Gush take Mentor to an entirely different level; but if there were a popular 2 mana sweeper, Mentor wouldn't be viable.
I've experimented with Terminus and it's great against Mentor and the rest of the field, until you run into a combo deck; sweepers are very hit or miss in Vintage so you can't just go ahead and start a deck with 4 of them. But if you could, Mentor would be in trouble. (while Yawgmoths Will would be excellent)