I think the UI in general needs a bit of reworking. The interfaces are very inconsistent. The depot windows for example are different between road and rail depots (look for example to how selling vehicles works in both). I also dislike how every dialog has a different size as well, and some windows (esp. the lines window) become bigger and bigger as the game progresses.
The UI should be consistent, and I think resizable windows is part of the featureset that is required.