In Hebrew writing, it is common to express the same idea twice but using two different phrasings or metaphors. For example, in Micah 4:3, the prophet writes:
and they shall beat their swords into plowshares,
and their spears into pruning hooks;
The same idea is given twice: instruments of war will become instruments of peacetime. Similarly in Micah 3:2, the idea of "fuller's soap" should be understood along the lines of "refiner's fire." With the refiner's fire, someone would bring a lump of gold or silver and the refiner would use fire to burn off the dross and purify the precious metal. Similarly people would bring their wool to the fuller and he would use soap to clean the wool and remove the impurities so that what is left is pure wool.
Perhaps the reason soap doesn't sound threatening is that the metaphor is misunderstood. The gold or wool being refined or purified is not an individual, but it is the nation itself. In verse 1:1, the oracle of God spoken by Malachi is spoken to Israel. Thus the question, "Who can endure his coming?" When the soap comes only those who are pure will remain. Certainly not the sorcerers or the adulterers or those who speak falsely or the those who oppress the hired worker, the widow, the fatherless, and the stranger, says Malachi.
Reading earlier in Malachi we see that the priesthood has been defiled; they offer polluted offerings, they have turned from God and refuse to listen to him, they profane God's covenant. Malachi therefore warns them that God is coming in judgment, and if they do not turn back to God they will certainly not be able to stand at his coming. For He is a refiner's fire who burns away the kind of dross that they represent. And He is a fuller's soap who scrubs out the kind of dirt and defilement that they represent.