Diacetyl reminds me most of buttered popcorn, which makes sense because the fake butter on popcorn IS diacetyl. It doesn't really remind me of butterscotch or toffee as much. I believe I pick up those flavors (when present) more from malt than from diacetyl.
Some say at very low levels, even though you might not be able to taste it, diacetyl might feel oily on your tongue, a "slickness".