Must haves 3

Low level magic is common: Oh yeah. If in some settings, you wonder how there can be an arcane caster in every adventuring party, in this one, you'll wonder why this farmer family doesn't have a potion of healing. Oh yes, that common. While still expensive, basic magic items will be buyable from pretty much every merchant.
    Why is that? Well, two things. I like magic items, I like wacky items that give you stupid abilities (I once gave a wizard a dagger that created a copper piece every time it would hit someone). For these to be available, magic how to be common. Also, magic items are fun to get for PCs.
    Might even give everyone a cantrip to use, once a day, chosen on birth, just to show that magic is in their blood. Not sure about what that would do for balance though...

Numerous sects: Without active gods, sects are sure to pop in everywhere. What I mean by sect is a religion that was dishonestly set up for personal gain (please, no debate about how every religion would be a sect with that definition). These add to this no-god world in making it more realistic (because, let's be real, such things do happen in our world, so definitely would in this one), bringing conflict (conflict is good for a setting), and setting PCs up to doubt every religion. Without that, they probably will not get the godless part of this setting, since they're used to the regular setting, with active gods.

Ninjas: Well... Out.

Samurai: I like the idea of the overhonorable fighter, but... it doesn't fit with what I want for this setting, where everyone will be mostly out for themselves... Out.

Civil War: Why? CONFLICT! Also, I don't want this to be simply a government getting overthrown, I want this to be a conflict set up around a moral issue, like what we had in the Marvel Universe, or in Skyrim. This creates moral dilemmas, forces players to roleplay a bit more ("I think they're right, but which side would Markas be on?"), and makes the conflict one where no side is right in the absolute: it's easy to pick a side when orcs are attacking a village, or when a necromancer is raiding graveyards, but in this case, just choosing whose side you're on will be one of the toughest parts.
    Also, another thing about civil wars is that you fight people you used to be friends with, maybe even family members or lovers. What does it mean? EVEN MORE CONFLICT!

Vikings: Well, not horned helmet blonde axe wielder that come in flat boats. But I like the idea of a culture living on an island, far from "civilization", and whose only interaction with it is to come and raid it every now and then. But they won't be like your regular vikings. They might not even be human. Maybe from the underground? Another plane?

No polymorph/no druid: I HATE POLYMORPH! I also don't think druids deserve a special spot in a world where gods don't exist and everyone share causes rather than gods. There you go, I've said it.

Lycanthropes: This might sound weird right after no polymorph, but I want to keep lycanthropes. In a world where polymorph is unheard of, they're actually special for being able to shapeshift. However, it won't be under their control.

Vampires: Well obviously there will be vampires, this is DnD! I don't expect to give them a particular place though.

Evil Elminster: In the Forgotten Realms, the most powerful mortal is Elminster, an archmage. He is good and keeps evil at bay, even though he doesn't intervene much (he's busy getting it on with a goddess). However, in Dragonlance, the most powerful mortal (from midway through the War of the Lance up to his death) is Raistlin, an evil, evil archmage. However, he keeps to himself and is not simply trying to destroy the old world (well, he is, but he doesn't do it, it's complicated). He stays on the side and doesn't intervene much in the mortal world. And, well, Raistlin is my favorite character ever. I need one like that. It'll probably be a lich. There will probably be a whole religion dedicated to him. But he won't intervene much, he'll stay busy with research (evil research), and may be leading a country a la Dr. Doom: not an evil country, and it's not there to conquer the world, but just so he can call himself King or whatever. I have a name for him: Dalam. It's the inverse of "Malade", french word for sick. I'll have to admit it's taken from a buddy's username on a few online games. But I like it.

An island where criminals can live: Think Tortuga. That's where you go to contact criminals. That's where criminals stay. And the law doesn't go there. Why? Oh c'mon, you didn't see it coming? Obviously, it's led by the Evil Wizard. I wanted him to rule a country? There you go. He's also the only one who can keep such a place from burning to the ground.
    Why did he do that? He would have liked to have a safe haven for himself when he was younger, and decided to create one for the next generations. Plus it serves him, like that no do-gooder comes to bother him (they'd have to go through this whole island). People most likely donate in his temples too, to keep his wrath away from them (remember what I said earlier about him being the center of a religion?).

Dinosaurs: Like them. I want a culture that has domesticated them. I want a half-dragon T-Rex leading a group of dinosaurs. Dinosaurs make everything great. The problem is, with such beasts walking around, how can regular animals survive? The cheap trick is to have them live on an island separated from the rest of the world. I'd like something more... elegant. However, I don't have any idea yet for that.

POKEMON: Yes, guilty pleasure. I've done it quite some time ago, and seen people do it since: creating DnD stats for different pokemon. Now, I don't want them to play a big part in the world, or to even be separate from other creatures: I'd just like my PCs to face a Bulbasaur eventually. It won't be called a bulbasaur, more like a Flower Toad, just like a Pikachu would be a Thunder Rat, and a Geodude, an Armed Stone. This is a very childish thing to do, but screw you. I like pokemons, and they're in.

At least one “civilized” nation led by a member of a “barbarian” race: To show the lack of racial difference, mainly. Just like a black president of the US shows that large-scale, official racism is a thing of the past (once again, I don't want your ideas on this, thank you), having a Bugbear baron of a "good", "human" kingdom would drive the point home.

BONUS!: Mutants: I like the idea of having characters who physically mutate as they gain power. It would be an NPC thing, but I'd like a faction whose member get mutations through a tainted magical source. Some positive, a great deal negative. Some major, some simply aesthetic. That does give me ideas for the type of faction it would create: if you get positive mutations, it's because our "God" likes you, if you don't, it's your damnation.
    Maybe it could be put together with the "Viking" raider culture I was talking about earlier on, to make them different from regular Vikings. Plus, the "tainted magical source" could be a reason to go raiding, as they need something from the "main world" to feed it, and they can't simply trade, as the main world attacks them at first sight due to their mutations.

BONUS 2: Traders: Those of you who know Giant In The Playground, you'll call me a copycat. I guess I am. The idea of an entire race being only known in the main world as traders, who bring some exotic wares, who are shrouded in mystery, is very alluring. I was thinking of calling them just that, the Traders, just like the Giant did, but you know, that would be stepping over the line a bit.

Here we are. 3 massive posts later, step 2 is done. I hope your list is a bit shorter than mine, and you cut a couple more ideas than I did. To complete Step 2: Define you must haves, you must have (hehe) your list, with a bit of explanation for why you want each of these things, and just some of the thought you've put into each, and how they interact with one another and with your defining aspects. They won't be the only things in your setting, and you will cut some out, but it is, I believe, an important part of the definition of your setting. Go ahead. Do it.