The main difference is no slot, which adds a lot to the effectiveness of a slat, allowing airflow to remain attached at greater angles of attack (slats work mainly by increasing the stall AOA, so more lift can be generated).
Krueger flaps work like a hinged leading edge flap rather than like a slat. An airflow separation bubble is formed behind the Kreuger flap by the angled section which unfolds as the flap extends, so although it looks as though it didn't ought to work the air flowing behind it behaves very much as if there is a complete lower surface there, reattaching further aft. Crude looking but effective. Basically a Krueger flap increases wing camber and, like a slat, increases the AOA at which the wing will stall.
Boeing combined the slat with the Krueger flap and came up with the variable camber flap, as used outboard on the 747 leading edge.
The glass isn't half empty, or half full, it's twice as big as it needs to be.