To the question, not necessarily the "poll," of the thread, MY definition of a "Vanilla" setting would be:
Includes all of the base assumptions of the Player's Handbook of a given system. In the case of a D&D world, at minimum, that is Humans, Elves, Dwarves, Halflings, Fighters, Clerics, Mages/Wizards, Thieves/Rogues. I would say you could get up to...say, 10 species and 12 classes before you begin venturing outside of "vanilla" territory. There is a degree of cultural presumption attached to many of these elements: Elves live in the forests. Dwarves are from the mountains and great miners/smiths, Humans are, generally, the most widespread and diverse peoples -but the most limited by lifespan.
MAGIC exists and functions to astonishing physics-breaking reality-altering effects and may be accessed in various different ways. The "vanilla" setting for D&D, I would say includes a minimum "spread" of 6 spell levels. Like most other elements, there is a dial from low-to-high magic which may include significantly more powerful or less powerful magic in a given region, if not the whole world. But the average, what I would call a "vanilla baseline" would include things "Sleep/Fireball/Teleport" mages, "Cure Wounds/Dispel Magic/Raise Dead" clerics, "Entangle/Call Lightning/Wall of Thorns" druids. Who/how many magic-workers you have, the spread of their power, how that power is manifest (or not) in political power is all adjustable a great deal and still remain a "vanilla" setting. Non-vanilla magic would be adding in things like different magics, e.g. Psychic powers, or divergent systems beyond a standard spell progression/slot system, e.g. "Blood Magic" or "Spells via magical Tattoos" or "Spells only work at such times/under X moon or Y star-alignment" that kind of thing.
A vanilla setting assumes the existence of fantastic creatures and monsters, beyond those supernatural beings included as character species, e.g. griffons, mermen, goblins, trolls, etc... This does not necessitate the inclusion of EVERYthing in a given Monster Manual existing in the world at once. Nor does it mean every monster in every creature compendium put out across a single edition (let alone more than one game/edition) must have some place in the world. But it does, in nearly all cases, include Dragons.
Given the above two criteria, the vanilla setting functions, mostly, as we would expect in the real world. The world is round. There are stars, a sun, a moon or moons. There are days, years, a -generally standardized- turning of the seasons. Time progresses. Gravity, force, the speed of light are all actual things that function as we understand them. Part of the entire point of the existence of beings and energies being "Magic/magical" is that they break, or exist in spite of, these things we recognize from our reality.
A "vanilla" setting is most likely to take place within a feudal monarchy, or similar geopolitical regions we comprehend/recognize from the real world. There are the not uncommon addition of things like "temple complexes/theocratic hierarchies" which may or may not control their own region/nation or "wizard-led/-ruled societies," and other strongholds and organizations that encompass the base assumptions of species and class: thieves' guilds, druidic organizations/"circles," monk & paladinic orders, elfin woodland strongholds, pastoral halfling villages, and the like.
A vanilla setting is one in which there is a definite sense of who/what is Good and who/what is Evil. This may be individual creatures, entire nations, or esoteric threat (demonic invasion, evil wizard going to take over the world, etc...). In nearly all examples, whatever the established 'villainy" is seeking to overthrow the "goodly" civilization/kingdom/world the heroes care about and (presumably) come from. There is room for some ethical/moral "greyness," Neutral-aligned characters, the noble-hearted thief or hero-in-the-making mercenary warrior, double- and triple-crossing villains and heroes, and such like. The possibility (and realities) of betrayal and redemption are, after all, quintessential to myth-/story-making. But broadly speaking, on a "world" scale, there is a clear demarcation of a "good" side and a "bad" side.
I'd say, any and everything beyond that...begins to play with the flavor. Want to add guns as a basic technology of your setting? Not vanilla. Lasers and spaceships? Not vanilla for a fantasy setting (yes, yes, I know. "But Barrier Peaks...!!!" I know. But I'd submit that is a relic of the pulp fiction elements of the game's roots and we..."know better" now, as a genre.). Want to add quadruped talking animals as player characters? Not vanilla. There are Sixteen Schools of Sorcery and only with the tattoo of the a given school can you cast spells/use magic of that type? Not vanilla (though could certainly be teased out of the vanilla assumption of Magics). The world is a double helix and there is no way to get from MortalHumanland to ShiningGodsheim without passing, physically, through at least 12 other "rungs" of the celestial ladder first? Not vanilla. What is called a desert is desolate expanse of marshmallow fluff? Mountains are giant tacos mined by the jalapenomes for the uber-precious veins of cumin and cayenne? Not vanilla.
I think that covers my "baseline" for what constitutes a "vanilla" setting. As should be evident, there is a great deal of leeway in some areas. Less so in others.