Tomte has a great post there, but one thing to note: your offhand has a 50% proc rate penalty over your primary hand. So a weapon in the offhand will proc half as much. This means your offhand weapon represents less than 50% of your total proc potential as stated. Total procs over a fight will approach 66%/33% primary/offhand.
This means, for instance, that a proccing weapon which beats a DPS weapon for threat in the main hand may not beat the same weapon for threat in the offhand. Which can be quite important.
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