I can tell you what was wrong with Baldur's Gate: it was simply TOO big. I suspect it's why BG2 got cut down in size a lot compared to BG1. While it was great to explore the vast outdoor areas of the Sword Coast, there were too many areas with not a lot really going on that you just wandered through killing stuff. In terms of actual substantive content, the number of outdoor areas could easily have been halved without losing anything.
DD got the balance right: the sense of scale enough to make it seem like a real world, without there being too much of it.
Not that I didn't love BG. I'm just not blind to its flaws
Perhaps, yes.
I think a good RPG you need to have the feeling you are free to explore everything, and be allowed to, but realy encouraged to follow a couple paths without them being mandatory. A feeling that you are hinted in what its best for you to do (*) but you dont necessarily need to go there imidiately. (Its a hard balance to acomplish i'm sure)
On the other hand the side-quests and free-roaming need to be fun without making you stockpile 50 simultaneous quests and need to go to point X to POint Y and feel lost in geting 20 quests in-between. Free roaming is fun if you can explore 100 square miles and have interesting stuff to do there, but no so much fun when you can explore 1000 square miles and feel lost with so much to do.
Not sure if i was clear, but as i said before, its realy hard to explain that balance that makes me like an RPG.
(*) (like in D1 you KNOW you need to visit the false divine one in the castle with an invitation, but you are free to go there when you feel its a proper time, or when you know you need to go to the dark forest because some quests point you there, but you decide that first you want to check whats in the sewers of verdistis. There is a choice there, an exploration, you dont feel lost.