Red has come out of spoiler season as the early winner. It has at least 3 cards I'm confident will see some amount of play, and another three that are likely.
Soul-Scar Mage, Harsh Mentor, and Magma Spray are all locks to see some amount of play. Even if the Standard red decks end up not being good enough, Magma Spray slots into Temur Tower and Grixis Control, Soul-Scar will just get better as more burn spells get printed, and Harsh Mentor is likely strong enough for both Modern and Legacy as a one-sided Eidolon of the Great Revel.
Let's focus on Harsh Mentor, since if any red card is going to end up as a long-term player from the set, Mentor seems like the one. While a 2/2 for 2 isn't going to excite anyone, it means it doesn't die to ping damage and can attack into random 1/1s. His ability, on the other hand, is top-notch, and truly scary for slower decks without much in the way of removal. In older formats, fetchland mana bases and the prevalence of utility lands means you'll never be without something to hit.
Modern: Fetches, Ghost Quarter, creaturelands, Horizon Canopy, Mishra's Bauble, Engineered Explosives, Eldrazi Displacer, Walking Ballista, Spellskite, Scavenging Ooze, Expedition Map and the bulk of Affinity and Lantern Control.
Now let's look at the primary drawback of Harsh Mentor. In some matches he's a situational 2/2 that's not going to accomplish anything. Is the Saheeli deck going to really care that you played Harsh Mentor? Not particularly. Had he hit planeswalkers as well that'd be a different story, but it's far more of an annoyance for decks continually using abilities over the occasional activation. Against Mardu for example, it makes utilizing Heart of Kiran on defense less of a sure thing and also turns Walking Ballista into a liability. Trading Mentor for a pair of Ballista counters and 4 life isn't bad at all.
Looking at the other decks on the cusp of making it big: B/G Energy gets hurt as well based on all the various abilities it utilizes from its creatures. Dynavolt Tower is obvious, but also takes care of the Mentor with only one activation, which isn't a whole lot of pain. Still, you get a body and an early attacker that trades off later for some damage unless they use real removal.
Still, there's not a great place for Harsh Mentor in Standard right now, though the outpouring of solid red cards is helping with that. Utilizing Soul-Scar against Mardu to keep normal enemy blockers in check and Mentor to ping if they go to crew is a nice little one-two punch. The key to Mentor is that he feels like a lock piece rather than a burn creature. You play him to help constrict and limit options for your opponent, unlike Eidolon, which is almost a guaranteed 2 damage if you play it on turn 2.
In Modern, though, that's when we really get cooking. Playing this on turn 2 puts a whole lot of decks in an awkward position. Unless you have the Lightning Bolt or Path to Exile ready to go, it inhibits your turn-2 play in a serious way and either gets a bunch of free damage or forces an awkward mana play. As I mentioned above, this assumes the answer is ready to go. For Affinity, it's very possible they just won't have one, and suddenly all of their best cards cost a whole lot of life to properly utilize.
What this also does is shore up the turn-2 play for Burn with something that doesn't need to go into combat. Previously, Eidolon of the Great Revel filled that spot, but now we can back it up with another play.
Even if Harsh Mentor doesn't quite make the cut in the main deck of Burn, as a sideboard option it's going to make life miserable for a few decks, and force them to always have removal at the ready. Have fun, Lantern Control players!