To get rejected for heroics, sometimes. Earlier today I hit 80, and then promptly joined a H VH group. I had trouble with aggro early on, which turned out to be due to a low axe skill (I got a new tanking axe today). That sorted itself out with time. The bird boss decided to get windfury and more windfury, or something, because I lost 20k health in about 2 seconds. I died.
It was a fun run though. I got the tanking pants. I wonder how many tanks just exploded with jealousy.
My paladin got into the tail end of WG and then spent a lot of time trying to find people to fight, only to run into three rogues at a time, while in healing gear (which I didn't realize at the time). Still, I got the fishing daily done, got the victory daily done, and then for no apparent reason tanked H Nexus.
I tried to do the same on my warrior. Someone got mad about my gear and voted me out of the group. I'm pretty sure it was the person being advertised as a pro healer. If he only wants overgeared tanks, why bother advertising that? I've gotten by with terrible healers just through pure overgearing, and of course being awesome. Anyway, I got criticized for being in all blues and told to do regular instances some more. I'm not sure how that would reduce the number of blues considering only blues drop in regulars. No one seemed to care that I am defense capped and my pants aren't blue.
I think the other purpose will be sink for mats that I can't seem to sell. An epic gun from my paladin should do the trick.
 I just tanked H VH on my warrior. Some pulls were messy and the first boss killed a mage, but no wipes, nothing all that close. Admittedly my gear is a little bit better, mostly a new gun from the previously mentioned mat sink.