We've all met them, the players who play the same class and spec as you but just don't do it right. It could be gear, talents, glyphs, gems, enchants and playstyle.
All of the first ones are quite easily fixed if the person isn't daft and as long as you don't use a totally un-diplomatic approach to tell them what they've done wrong. The last one though - playstyle - is alot trickier.
I don't claim to know much about any class except priest healing (and healing in general to some extent). And when I see a priest that doesn't do it right and if I know the raid/part is suffering by it I just can't keep my mouth shut. The problem is, what am I gonna say? I could simply say "hey, you're not doing it right, use this and that spell instead" or the like. But will it actually improve anything? I am not so sure.
The problem is - what is someone doing when they're stressed out? When they can't think about their next move it has to come naturally to them. This is what pvp is all about for instance, but it is needed in normal raiding too to some part and in heroic raiding to a big part. If I use the wrong spell at the wrong moment, the raid could and most likely will wipe. Every class and spec has to think about this but I think to healers it is even more important. Of course no one can stand in one-shot mechanics without being killed (except for nasty paladins) and therefor have to react quickly to the changes of the environment. The thing about healing is you usually have to think about all these environmental changes for yourself and for the rest of the raid. A dps or tank doesn't have to think about what another tank or dps is doing right now, just hope they're doing it right and do their thing right. But the healers job is to make sure everyone can do their job right. Ok, some dps have to tot/md, some tanks have to taunt but healers have to keep an eye on the mobs, the players and the environment at the same time. This means it is easier to be stressed out as healer than any other class (a tank who has to pick up adds comes to a close second though).
I am sure many won't agree with this. But one can put it into simple numbers. In a 25 man raid a healer has 25 targets to keep track of + not standing in fire. Tank and dps usually has 1 + not standing in fire. Quite the difference. But enough about that and back on track. What do people do when they're stressed?
Usually not the right thing. I remember not so long ago when I started healing I used to fall in nab-traps all the time. Taking your time to choose between slow big heal and fast small heal when the tank is near death. Choosing the wrong one and starting a big slow heal when the tank is near death. AoE-healing when there aren't enough targets to benefit from it and vice versa, not AoE-healing when too many are taking dmg. Not using cooldowns when they're needed and using them when they're not needed. Casting them on the wrong target. One favorite is of course spam healing yourself because you've forgot to mark the tank/whoever needed heals.
The list can go on and on. They usually compile to two issues - using an ui that doesn't aid in healing and not being totally familiar with your healing. Me targeting the wrong target has long since been fixed by using Clique. Using macros and the like to target is also helpful.
But chosing the right spell at the right time has only been helped by practice practice practice and even more practice. First time you encounter a new fight you can apply all your old knowledge all you like, but there still will be moments after a wipe where you think "oh ok, it works like that, well then I have to use this spell even faster".
A good example is Fusion Punch on Iron Council hardmode. Whenever I'm not healing I am telling the assigned healer "Ok, Fusion Punch has to be dispelled asap. I don't care what you do, drop it and dispel. Asap. Ok? Asap. Not like after one second. Start dispelling before it lands". Still it usually doesn't matter how many times I say it, if the healer doesn't see for his own eyes how frickin asap it has to be he won't do it.
And now to why I bring this up at all. We have a great priest healer in my guild who for some reason looks up to me as her healing guru. I mean great as in really nice person and really interested in improving her healing skills. To me it is interesting to see her do all the old mistakes I used to do and try to improve them for her. I have introduced her to clique and other features which really helped my healing. The problem is of course that she might give them a try, but when she's stressed out she will revert back to her old style of healing with the old mistakes. And if I tell her patiently and kindly to do it in another way over and over she will only become more uncertain and stressed about it. If she doesn't think she can improve she never will, no matter how many good ideas I throw at her.
So what is the difference between me and her?
I had no one to look up to when I healed. I did my mistakes but I had no one looking over my shoulder saying I could do it differently. I was for a long time the only active raiding priest in my guild and no one had more knowledge about it to give me any hints about anything. For some reason this turned out to be more good than bad. Having to figure it out all by myself means I can actually say I really know how to heal and know when I do something wrong. My healing is far from perfect and perfecting it is what has kept me going all these years. But the most important thing is I don't expect people to think that I'll do things badly.
I think for healers, letting people learn from their own mistakes, without it resulting in too much hazzle for the rest of the group, is the best way to improve their playing.