Mama Dolce's seems like it was only half-implemented--it's important enough to have a unique radio station, but there's really nothing of significance there. And there's all kinds of little details that make it seem like it wasn't finished--there's a switch in one room that does nothing; a "sewer grate" (actually looks like a really small patch of grating) stuck into a cave wall that can be opened and closed, but also does nothing (and doesn't change appearance when it opens/closes, either); the locked gate in the loading yard that requires a nonexistent key to open (and the bit beyond the gate is unfinished anyway); and the strange fact that the interior doesn't look a thing like the exterior, is rotated ninety degrees, and, if you look at the world map while you're inside, is located somewhere completely different than the exterior (to the west, on the outskirts of DC).
There's also Drowned Devil's Crossing, which has a really neat name for a random collapsed bridge in the middle of nowhere.
Alexandria Arms, the hotel by Arlington Library, has an elevator marked "INACCESSIBLE" in the front lobby. I always wondered why they put that in, considering there's no way to use it. And it's a pretty big building to just be a generic raider hideout. Springvale School at least had some interesting backstory through a terminal, and the ant tunnels.
I know none of these are unfinished quests, per se, but I always thought these locations seemed more significant than they were--like there should have been a quest or something associated with them.