Looking at this thread I see many of the same points I would make in describing
"what works" in my own homebrew. "Style of Campaign", "Personalization" and "Possessing Greater Knowledge of the Campaign than the Players"...
The devil is in the details as they say and I did a metric ton of research for it before getting into it in earnest, particularly on how I wanted it to look. I've downloaded probably thousands of images of villages, ancient towns, structures of all kinds, various terrain, stylized architecture both fantasy and real-life images from around the world. I spent many an hour cherry-picking certain elements here and there to build a baseline look I wanted for the various city-states in my game. For example, one of my cities I amalgamated elements from a French Village I liked for the garden elements, pinnacles of stone similar to those in China, organic flowing structures similar to a fantasy painting I downloaded, connected by bridges of my own design and then gave it all a flavor that felt appropriate for it's setting.
I find that once I create the "look" I'm searching for, the culture, social structure, government, ecology, adventure hooks just seem to flow from that baseline vision. I try to keep human civilization interesting, even extraordinary (in capability), but always
believable. NPCs play a big part in keeping it real. Population centres are generally smaller than you'd find on many other fantasy worlds - especially FR. Even the largest city-states have no more than 35-40,000 people in them. Many of them are just well-fortified towns of 16-20,000. This gives your home-city a more personal feeling I find and the more you can make the players care about their home, the easier it is to motivate them to do something for it.
What I like to do for NPCs is jot down, in just rough point form, events that *will* happen to them in certain intervals in the future. Perhaps they have a child or perhaps a prominant NPC they rescued has turned to drink and violence from his ordeal. Just things that ground them to the campaign and can be points of discussion when the PCs meet up with them again. And I try to do that on a broader campaign level too. Things continue to happen in the world even if the PCs aren't around to witness them.
But what I think is one of my favorite things to do in the campaign is what I like to call "unintended consequences". What happens after the adventure is over and the PCs leave for new adventures elsewhere? Did they just touch off a domino effect with their efforts that they did not anticipate? Did they leave a huge power-vaccuum begging to be filled? Did they unload a metric ton of gold into an economy unable to handle it? Are the guild masters they rescued in any better position to defend themselves after the PCs have departed? Will there be a civil uprising against the local authority for "allowing" a BBEG to do so much damage in the first place? Is there collateral damage the PCs need to answer for? Is someone looking for some payback? Things of that nature...
There's certainly more I could talk about, including introducing "fantastic" elements to a campaign in moderation so that they have greater impact. Fleshing out your major races to make them more believable - eg. Fire Giants IMC have a rather large empire on one of the southern continents and are known to appreciate great sculpture (stone, metal & glass) and (though rather violent) "theatre". It's a society that built around fire, stone and iron like we are drawn to water.