A) Deal damage; or
B) Take damage
to gain rage. Since you are ICC geared, you wouldn't be generating enough rage to maintain threat. By throwing on DPS gear and staying crit immune you'll generate enough rage to keep the baddies off the rest of the group.
If they have a problem with that, ask them if they'd like to tank and let you DPS.
Really, if it's anything before H ToC5, you can Fury Tank it with the same effect with a halfway comptent healer. In my Fury spec I'm only slightly behind in armor and way above in HP where I was when I first started tanking (successfully) UK, UP, and Nexus. Add in bloodthirst for some self-healing and the only issue is crits, but even those are rare given the fights last ~25 seconds and I can even take those better than when I was starting to tank due to the much larger HP pool.
For guild apps, as part of your app proactively explain why you gem/enchant/gear the way you do, even though it's not ideal, it's for a reason. Be proactive about it, don't bother waiting for them to ask you, especially if it's a guild raiding above your current tier.
You'll show them you understand the game and what is best, but also that you're willing to adjust to the situation as it is. Also make it clear you'll regem/re-enchant if in a raid situation you don't need to worry about clueless DPS going full out and not bothering with threat management.