Your gear doesn't affect your groups ability to do things that horribly much in wrath. Okay... if you have crafted blues it doesn't. If you are wearing level 70 greens at 80 then maybe it will :P.
Anyone can tank heroics. I know I've healed enough low hps tanks. Heck a tank with almost no hps can still -speed- tank heroics if his dps is good.
Anyone can dps heroics. This means ANYONE AT ALL if the rest of the group is median skill level*. This means anyone with 1k+ dps if the rest of the group has the wow population mean skill level.
Anyone can heal heroics. But you'll have to drink a lot if their gear is low.
I hear all the time people talking about gearscores etc via blogs. I have never been asked about my gear score. I have never heard anyone talk about a gearscore in game. And I've never seen anyone inspect another persons toon and quit party.**
I healed random heroics in blues and greens. Heck three pieces were below iLevel 150. Gearing up in my mind only means "making this less stressful and more fun". I -don't- like being mana starved(aka: below 80%) most of the instance. I -like- boredly letting juvey do all my healing. So I gear up for heroics. But that doesn't mean I needed to do so before running them.
Oh and I've never had to end a party because we didn't have the ability to finish the instance. Only because people couldn't play nice with each other.
Gear matters very very little towards your ability to beat content.
Gear however can quickly make your skill irrelevant. Oh you still might have wipes and slow down things...but with top end gear you -can- just sleep roll your face through eventually completing heroics.
Disclaimer: This kind of post... not my strong point. But the topic was on my mind. Everyone knows this... so more of a rant thing.
*AKA: the middle value on the skill range... not the population average.
**The only possible reason I can see to do this anyways. is if your tank isn't defense capped.... because please... anyone can get defense capped... they just might not have any health/threat left after they do so.