It has been explained to me that the main difference between limbo and another dream level is that once you are in limbo you forget the difference between a dream and reality. It has been shown between Cobb, Mal, and Saito. So why is it that when Cobb and Ariadne go looking for Fischer, they know perfectly well their mission and who they are finding, while Cobb forgets his mission for a while when finally finding saito?
The very short answer is that dropping into limbo doesn't immediately affect your memory but that the time-differential ultimately results in you accepting limbo as the sum total of your reality.
Cobb makes it clear that after you've been there for a (subjective) lifetime, your original, waking life begins to seem like a half-remembered dream.
COBB: Saito-san, when you wake you might not even remember that we had an arrangement. You’ll have forgotten this world. Limbo will be your reality. Lost there so long, you’ll have become an old man…
Dropping into Limbo “Manually” is to naturally keep sleeping and going deeper...If you do you won’t forget. But if you DIE you “Automatically” get sent to limbo, if under heave sedation, then you won’t remember because you died. So when you awake, you won’t know that ur dreaming, you’ll probably just think it’s normal.