Quick answer

If you are not hit capped in TBC , your damage is also your threat as a tank, so every miss is a direct threat loss. For a protection warrior, a missed hit means you lose the damage/threat from that swing or ability and also some rage generation, which can snowball into noticeably worse threat.

How much does it matter?

A rough way to think about it is:

  • Melee raid bosses in TBC are level 73 , so the common target is 9% hit for yellow attacks against bosses.
  • If you are below that, your threat is reduced by the share of attacks that miss, and the practical loss is biggest on high-threat abilities and auto-attacks that help fuel rage or resource generation.
  • For tanks, especially warriors, being under hit cap is often acceptable if it lets you keep important survivability stats; threat is usually managed with gearing, rotation, and raid support rather than chasing hit at all costs.

Practical rule of thumb

Situation| What happens to threat
---|---
Small miss chance| Minor, but still noticeable over a long fight.
Missing key abilities| Big threat drop, because those buttons are high value.
Low rage / low resource class| Misses hurt more, because they also reduce your ability to keep generating threat.
Well-supported raid tanking| Missing some hit is often manageable.

Bottom line

If you mean “how big is the threat loss from being not hit capped?” , the short answer is: it scales directly with your miss chance, and for tanks it can be very noticeable, but it is usually not worth sacrificing major defensive stats just to cap hit in TBC.

If you want, I can also give you the exact hit cap numbers for TBC tanks by class or a threat priority list for warrior / paladin / druid.