Some ability to loiter and choose targets via algorithm. Mines can't move, and guided missiles are spatially and temporally bounded in what they can target.

There have already been cases of "autonomous" targeting, such as in 1982 when Exocet missiles fired by Argentine aircraft at a British warship selected a cargo ship instead (possibly due to countermeasures used by the warship). In this case, the weapon hit a target not intended by the firing unit. Of course, unguided weapons do this all the time...