Connection and verticality. Flat maps tend to set a repetitive tone. The maps that tend to succeed for years onward is due to networks of connections that allow for flexibility. Industrial 17 reached its maximum potential with that level of amplification.
D47 V2 is my favourite map and I think this is mostly why, the trainyard in D47 is imo the best singular area in any HL2RP map ever.
It's a complete middle ground between all the factions in the server. Citizens have access to the balconies overlooking it, and can sneak through into the yard - A CWU warehouse is on the far side of the tracks to them, and the canal block and Nexus sit on opposite ends of the trainyard.
There's a half dozen if not more ways to move through the train yard from one area of the map to another, some of which are hidden and it gives a way for Citizens to sneak into the canal zone, and for rebels to sneak into the city.
It also provides a very long sightline, so that a Citizen or CP on the city side might spot a rebel on the far side, and vice versa, but without any risk to either of them. It's not a substantial interaction, but given that a lot of HL2RP is about wondering what the other side is up to and character motives, it can make the server feel more alive.
Trains and the warehouses are also an obvious target for rebels which puts the action right where everybody can get in on it. And again with the verticality and long sightlines it's very good for positioning players in interesting ways where they can support each other and has the various connections to quickly leave and go into hiding.
By comparison I think C24 is a fairly weak map for layout as the slums were very clearly sectioned off away from everything else with very few ways to get in or out. The only real areas in the city were stores and given the city was structured around literally 2 streets everything that ever happened could be witnessed by 2 CPs on either one. Both entrances to the slums in both versions that Willard used led out onto the exact same street which meant you could effectively close down the slums by just sitting on that road.