No, this post isn't just about Skyrim. :-) Path of Evil was an open world on the scale of Faerun, and Crimmor is an open world on the scale of the city.
Open world games have something to keep in mind that more linear games do not, the density of points of interest in the world. Think of the overland maps in SoZ, and imagine if there were only 3-4 places you could go on them. That would create two problems. First, it would be boring, and second, you'd spend lots of time just walking from between place. Now imagine the opposite, several hundred of places you could go jammed into the overland maps. That creates two problems as well. First, there's a logic problem, "You mean there's a dungeon/cave/ruin every 2 meters between here and the next place I need to go?", and second, the player may very well explore all those places on their way to the destination, with the "saving the world waits for me!" problem.
There's a happy median to be found somewhere in there. Open world games need to provide plenty of points of interest on the main land map, but not pack them together so tightly that they run into the problems from that.
In Path of Evil, the points of interest outside the cities weren't always as interesting as they could have been. Most of the quests centered around cities, or areas you could travel to from a city without needing to move to the world map. There were a few quests you could wander into on the world map, but for the most part the world map locations were just random dungeons of no particular interest to the player other than for what loot could be gotten. I think ultimately the campaign suffered some for that. I'm not sure what readers might think of the density of locations on the campaign world map, but I think the density of interesting locations was too low.
Naturally, this perceived weakness is something I'm trying to improve in Crimmor. It's making a lot of work in order to fill out the gameworld with enough to do. Currently things are going relatively slow. I've been playing some other games (sacrilege!) and not been super motivated (this is normal in the middle of winter for me). I havnt posted to the Bioware Social thread for Crimmor because there hasn't been anything worth showing, just some minor sidequests in existing areas, and some frustrating work on my commoner ai project that seemed to go nowhere. I'm considering redoing much of the work on that and getting a cleaner start based on what I know now.