Older edition adventures and supplements are far ranging in the situations they depict, yet retain common elements to make them useful to players and referees of older editions. The most obvious one being the statistic presented in a terse format compatible with one of the older editions of Dungeons and Dragons.
Compared to other types of adventures and supplements there is a lack of city supplements. A product that details a large settlement for use in a campaign. One of the more famous is Vornheim and it has it's own distinct format. One reason Vornheim is unique and a success is because it more about how Zak runs cities than a physical description of the place. And as it turns out many people found how Zak runs a city is useful to running cities in their own campaign.
So.... which lead me to the question asked in the title of the post.
What to you want out of a city supplement written specifically for the OSR? Other than everything stated for older editions of DnD.
Off the top of my head here are some things that could be in such a product.
A physical description of the settlement, the traditional map with a number key followed by a description of each locale indexed by said number key.
How the author runs cities in his campaign, guidelines, random tables, etc
What I been called a sandbox of NPCs. Basically a list of important and interesting inhabitants with descriptions and just as important how they interact with each. Referees can use this for campaign focused more the adventure that arises out of meeting people and dealing with the complications that arise from that.
Basically what would make a city supplement more useful to you as a player or referee of older editions compared to say Cities of Harn, Chaosium's Thieves World, etc.
Finally if you like Vornheim or want another good resource for refereeing a city in a campaign I recommend getting Midkemia Press Cities. I think I was their first PDF customer as they just sold out all the physical copies. Not only I got it I use it. It basically three sections, one is an outstanding series of random encounter charts for cities. The second is what they call character catch-up, basically a series of guidelines and charts to find out what the character been doing in between sessions. The last is a series of city building guidlines and charts. I printed these up as three digest size booklets and they been working out great.