The D series modules all had that large scale hex map of the underground caverns, nearly all of which were 'for the DM to develop'.
Yes, D1-3 definitely had that issue. The first time I started a run with D1, one of the players wanted to turn right after the first encounter and go off the identified path on the map. He thought there would be nothing but traps down that path and wanted to try and find a way around it, otherwise, what was all that blank space doing on the map?
Those were some reasonable assumptions and the party went with it. I gave some hints to the contrary, but they were not believed.
Except for the first encounter, all the set portions of D1-2, which I had carefully studied, became useless.
It required quick thinking on my part for the various encounter points on the map, especially since I didn't want to throw up what amounted to a retaliatory wall for them not following the "path".
It also resulted in substantial delays because there is no really clear path to the Vault on either side of the map, the center path is the ideal way to get there.
I suppose I could flame D1-2 for not fully detailing the entire underground. After all, it did not have to include all those extra tunnels. It could have railroaded the players down one set tunnel. But those extra spaces, empty and without detail, were fantastic.
Still, the modules had the potential for literally years of gaming, if the DM wanted to take the time and develop all those areas.
Another resounding yes. I have spent quite a bit of time staring at the DM underground map for D1-3, imagining what would be going on down there.
The various peripheral encounter areas are not well-connected to each other. There is little that could be considered as routes between them and the Vault. Yet the random encounter table has drow merchants on it, and D3 has the Vault populated with various merchant houses. I always wondered who they were trading with or how they could possibly have made a profit given all the dangers imposed by the wandering monsters chart. This is a logical flaw in D1-3 design. It's not one I am going to criticize, though. D1-3 were great. So was Q1. They shared many similar design elements.