And I'm curious about the "never go into a dungeon raid". Do you enforce class balance at the table by leaving out certain characters from certain adventures? Are they allowed to use backup characters, or do they just not play that night?
I guess I'd have to ask why the PCs would choose to follow a lead that excluded one of them. As a DM, it's my job to create interesting adventures and plot hooks that apply to their characters.
I have preferences and certain boundaries about what I'll run (just like my players won't play Aces & Eights, no matter how much I beg), but even my campaign setting evolves based on the characters played in it. The historic personalities and "name brands" are mostly previous PCs, rather than some designer's Mary Sue. Enduring nations and orders are founded by PCs, they fell ancient empires, and permanently change the map. That's the trade-off for actually having to have a personality and bring it to life.
As you say, the social contract is king. I put a lot of effort into the story, when I DM, so I expect and attract players that also do so. That said, there are times we the group wants to kick back and have a low-stress game of stabbing orcs in the face and that uses a different social contract.
For "beer & pretzels" mode, I'll grab a module and use a published setting (either Greyhawk or Eberron, most likely) and no one will make too big a deal about little things ("Hey, wasn't the blacksmith's name 'Jim' last week? Why is it 'Greg', today?"). In that case, the expectation is that the players will make characters that don't feel out of place, but they can retcon some history in. Also, we're playing casual, so anyone who put too much effort into maxing out their character would be out of place.
Thankfully, I've never been part of a group whose primary joy was in optimizing their numbers to the point they couldn't turn it off. Sure, everyone likes a character who's competent at their intended role, but that's not the same thing. I don't understand that mentality. If the whole group is doing it, then whatever floats their boat; they just have different criteria for what's "deep" or "casual" than I do. If their group has players who don't want to do heavy math, or who aren't good at it, then there are probably some issues there.