What is a 1e feel?
In terms general game rules, a 1e feel is one where the mechanics have no consistency. Where every mechanic in the game is either a single percentage die roll off of a table, or else some arcane mix of obtuse and cumbersome mechanics chosen seemingly at random. The more table lookups you have and the more inscrutable the design, the more 1e it feels. Bonus points if you have two competing systems that accomplish identical purposes in totally different and incompatible ways. In this way, reading the game rules feels like you're perusing a magical tome of arcane secrets. The term
arcane truly is all encompassing here in every meaning of the word. It should feel like you can't change anything without collapsing the whole system like a house of cards, and also you should not be able to tell what the intent of the game design is or what the designer of the game intended. Quite frankly, you should be able to read the entirety of the game's rules and still have
absolutely no idea how to play the game at all.
Notably, healing and recovery should be almost overwhelmingly difficult, explicitly sacrificing playability in favor of "realism." On the one hand, this discourages players from always relying on combat to achieve their goals, but on the other hand the only way to even begin to overcome this hindrance is to include a cleric or other source of magic healing in the party. If combat doesn't feel like it will essentially always have a negative consequence on the pace of play, your healing is too good.
[Note: I put "realism" in quotes above because the same game also ignores permanent injury, which I think is necessary for actual realism even if it clearly makes the game unplayable. That is to say, I think it's disingenuous to call any recovery system "realistic" when there is no system of permanent injury or consequence to injury outside of what the DM arbitrarily decides and the existence of one spell and a handful of items.]
In terms of classes, I would say that all characters have all their abilities at first level. Those abilities may and often do improve throughout play as levels increase -- even going so far as to start out functionally useless at early levels -- but there should be virtually no brand new abilities tied to class level. Further, any abilities that you have should generally be relatively minor, or powerful but narrow, or powerful but with severe drawbacks.
Your "progression" in the game is solely determined by the XP you earn -- inherent HP, attack bonus, save bonus, and spell count but that's it -- and the treasure you find, especially the magic items. This is doubly true if the XP you earn is directly tied to the treasure you find rather than being a function of the challenges you overcome. The one exception to this design is the ability to construct a stronghold at high level and transition from purely an adventurer or mercenary into nobility, politics, state-building, armies, mass combat, etc. This major transformation of play at "name" level or at 10th level is relatively core to the 1e experience even if, IMX, most tables ignored it in practice.
The biggest ability of every character in a 1e game is the creativity of
the player, and the DM should be willing to entertain any creative solution to a problem. Towards this end, the design should overwhelmingly favor martial characters at the levels people actually play. This is because, aside from main force, martials don't have any ability at all except a lot of HP. So, martials get the best equipment draw, and that extends to magic item tables, too. At higher level, magic is much more potent, but high level martials are far more resistant to magic, have a lot of equipment which negates magic, and monsters will often get two chances to resist a spell (magic resistance). Magic is very, very potent, but between full Vancian casting (i.e., slot preparation), limited spell selection (often determined randomly), and severe restrictions on spellcasters, every spellcaster is a glass cannon... but only on a good day. The more abilities a character gets, the more penalties and disadvantages. This doesn't mean you give any nod to inter-class balance, it just means things are made with many critical flaws or drawbacks so that martials are effectively mandatory.
DMs are encouraged to make campaign worlds large sandboxes without a central narrative. Stationing a campaign in a town surrounded by wilderness, or presenting the players with a hexcrawl map are both deeply 1e play styles. There should be no central narrative to the game; adventures are what you find, are often disconnected or independent from each other, and are kind of just about what the players themselves want to do in the campaign world rather than what story the DM and players want to tell or what the player characters want to accomplish.
Save or die or single die roll deaths do happen, but... at least in the games I played in they were largely used (a) against NPCs by the PCs, (b) against PCs when they did something really stupid (doing nothing to avoid meeting a medusa's gaze), or (c) when the other PCs or DM could do something to keep the character going. I know some people who played meat grinder games, but that's just not central to what I'd call the 1e experience. Tthe game's ability to support a meat grinder game probably is essential to 1e, though. Still, enough of the games handwaved that away that I really don't think most tables honestly played that way once players started to actually roleplay their characters and not treat the game as a competitive DM vs PCs metagame dungeon crawl challenge (a la, Tomb of Horrors). I think that style of play has largely been subsumed by by modern video games which just do it better and on tap. I don't think it was a surprise that most of the people I knew who liked D&D but never played always assumed the game was full of save-or-die effects, while the people I knew who played the game regularly just didn't use that very often or only used it in ways that could be mitigated if the players were careful. That might be my personal experience or my own hindsight, but that's what I remember.
In terms of gameplay settings, the themes of the campaign setting and motivations of the PCs will strongly favor colonialism and will glorify noble station and feudalism in the same way that myths, legends, and fairy tales tend to do. On the one hand, this means the game can be focused on wide open, untamed, wilderness exploration. Spelunking ancient dungeons, discovering lost treasures, fighting fantastic monsters. High adventure reminiscent of the European and white American expansion into the American west or African jungle.
On the other hand, it's overtly human-centric (in some cases to the point of xenophobia) and is about expanding human influence in the world even if the characters (or players!) don't realize it. The PCs are the vanguard of human civilization taking over the world; the first steps to bringing order to wilderness. That's why the original alignments were Law vs Chaos (even if it was cribbed from Three Hearts Three Lions) instead of Good vs Evil. That's why you end up building a stronghold; your characters are expected to conquer a region and rule over it and extract wealth from the land. You'll oversee clear cutting of forests for building materials and establishment of vast farmlands to feed the settling populace as you use your armies to drive out the monsters and goblinoids formerly living there.
In short, I would consider the 1e feel to be the
core OSR principals combined with generally poor game design and overall cumbersome game mechanics. Plus some questionable gameplay themes when looked at objectively, even if the idea is just to emulate romantic fantasy epics, fairy tales, mythologies, and legends.
What do I think Frog God Games means by a 1e feel?
My guess is that Frog God Games is thinking more OSR principals and adventure modules that lack a connected plotline and can be dropped in anywhere.