It's funny you say that, ejamer, because the open world-ness is exactly what my least favorite part of the game was. I'm basing this off of memory because I haven't picked Lego City: Undercover back up to start over with retroactive yet, but I remember it being horribly empty, uninteresting, and not motivating to go searching for the collectibles.
I mean, for this type of game, I didn't find the open world an impediment to playing it, but it was at best an OK window dressing to the story missions.
I need to get started on this game tonight, so I know i'm not talking out my rear here.