In the case of heavy mitigation gear, warriors can still do quite a bit to crank up threat: hit and expertise probably aren't capped, and are excellent stats for ramping up our threat. In this case, I understand that hit and expertise contribute more average threat per second than paladin spellpower will, putting us at a scaling advantage.
In threat gear, warriors have already reached the hard caps on dodge-reduction and miss-reduction. Block value is the only avenue left for our scaling, and it's an inefficient stat compared to the previous options. According to the itemization posted back on page 2 or 3, it's also significantly inferior to the scaling for paladin's spellpower. In short, our potential threat scaling in heavy threat gear is somewhere between "terrible" and "nonexistant," with the noted exception that the 1.5-speed sword will increase threat in an infinite or extremely high rage scenario.
-Splug
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(Usually) Prot warrior:
http://www.wowarmory.com/character-sheet.xml?r=Illidan&n=Splug