It seems like my very first day at level 84 and those three LFD groups were bad luck. Since then most groups were actually pretty smooth. Besides luck there are two reasons for this, I think. First melees are f***ed in Cataclysm heroics. I can easily predict the challenge of any group based on the number of melee dps, and for some reason my first groups had a lot of melees.
You could say
0 melee: easy mode
1 melee: still ok
2 melee: really hard
3 melee: can be impossible
Second, I didn't use the 'random dungeon' feature at lvl 84, because I wanted to get into the instances that still had drops and not those I already ran at levels 80-83. Consequently I didn't have the 'luck of the draw' buff that increases all healing, health and damage by 15% currently.
Using the random LFD at 85, I was able to heal heroics pretty soon, and not trying to heal melees at any cost actually allows me to even enjoy the run. I'd say it is a 8 at my last post's challenge scale. It's really hard to keep everybody alive and the groups are far from perfect. By concentrating on the tank, the melees are more careful (when they reach 50% health and are not instantly healed). This way I usually don't run oom unless we fight two groups at once. What is extremely useful right now is the undeads' skill "cannibalize" which restores 35% mana over 10 seconds if I can find some human or undead corpse. Interestingly this works perfectly well in combat and has saved quite some wipes so far. I even ended up not looting prior groups before boss fights so that the corpses stay longer to eat. Yeah, that's almost some innovative thinking here in WoW. Who would have thought?!
This morning I recorded a heroic for you to watch. The full heroic is 1 GB and that's too much to upload for my taste. That's why you get a nine minute snapshot.
And if you're interested in my equipment/specc/glyphes, here's the video. I slightly upgraded the weapon, but otherwise it is the same.