Haven't had any horrible fail groups, however this is coming from a well geared healer....
Here's what I've noticed...in general terms:
Tanks: You get 1 of 3 tanks.
Tank 1 likes to stack stamina and talk about his/her +45k health. Then proceeds to get completely raped pulling too many mobs and blames the dps/healer (me).
Tank 2 is just a new tank, usually pretty friendly. However this only thing I've found is that this tank is usually friendly but not exactly attentive when you ask them things, or gives you a general response of "oh kkk, I got it now" then does the same thing again. It's not horrible, just please....if you're dying with a good/well geared healer then something is wrong, listen to others opinions...(Lots of DKs, don't act like you know everything. Warriors just as bad.)
Tank 3 is the good tank, the one with avoidance and stam :O! This tank usually has 38-45k health and doesn't really say much unless a guildy or something is in the run. He/She waits for mana if needed but usually goes at a consistent pace. (Sidenote: Haven't had a DK tank who fits this profile yet).
DPS for me has come in 2 sizes.
DPS 1 is good, pulls >2.5k dps and doesn't complain about shit like gear rolls or "people needing a certain item because its the only one they need". Rarely this person has the quality trait of elitist and proceeds to attack anything that isn't "pro".
DPS 2 is the opposite of DPS 1, this person is cocky or a shithead. Constantly trying to either justify bad dps (I don't take this game seriously, I don't play all day, etc) or justify being a shithead (pulling aggro off of a GOOD tank, pulling unneeded mobs). Now I don't usually say anything to the dps, however I usually give them the option of being a tank or dps...if you would like to tank please inform me and I can heal you. If not, please stop pulling aggro...I'm not healing your dumbass.
Now as a healer I don't encounter healers (unless I'm running around bored in shadow) but what I've noticed is that as a healer we see things two ways:
The group is good, the people are smart and generally know what to do...this type of group is headache free, I dont have to constantly switch healing targets and the tank keeps aggro. This type of instance is fun, because I don't have to work that hard...(sorry it's true).
The group is bad or 1 person is lacking. This group is not horrible, but again I dislike getting people who think they're the shit...or don't play by the general instance rules when pugging. Stop being a hero, don't talk smack, don't cry about loot, and play well. This group usually has at least 1 bad who complains, fails, or crys. I don't care, just don't call out a tank or healer because "they are a bad tank (unless they are) or bad healer (unless they are). If YOU die...it is your fault...especially upon pulling aggro or dying to something stupid. If I miss a heal on the dps because I'm keeping the tank up...that sucks...but its life. Rez ftw.
Thank you that is all...