In my guild forums there is a section called Rate My Toon! The idea is, as you may guess, to submit your toon for a thorough critique from guildies. They will inspect your spec, gear, gems, enchants, and glyphs and point out any spots that could be improved in order to maximize your character's capabilities, and as a result, the raid's capabilities.
Up until now, I had resisted linking my armory for critique. It's a combination of reasons: I am lazy, I don't like to be told I've made a mistake, I'm the only mainspec resto druid on the raid team, I feel relatively confident about my choices. But my guildies did have advice! My moonkin buddy identified a socket bonus I had met that could be ignored in favor of a pure +int gem (/twitch...gray text...), and asked about the Heartsong weapon enchant, which prompted me to discover Power Torrent. It's not viable until we get a more regular supply of maelstrom crystals - and I get a weapon worth enchanting with such - but I hadn't even known about it before now. (Embarrassing! I hadn't updated my BiS notes in months...) He even questioned my spec, recommending Furor over Moonglow, but I defended that by producing a few blog posts and guides explaining my choices.
In the case of my forum, the venue is obviously already an appropriate place to give advice, and by posting there you acknowledge that you are asking for it. Still, knowing the right tone for your audience is important. Although I've talked about the need to pay attention to messages, even when given with a combative tone, I would still have been annoyed to get a comment like "lolol - you look like a noob gemming for a +spirit socket bonus!" (/twitch...gray text...) Ophelie sums it up as follows:
Be clear about how you like to spoken to (for example, I’m very open to blunt feedback, but I don’t let anyone raise their voice or swear at me).Hell, even though all the comments on my armory were respectful and helpful and I had explicitly asked for input, I still found myself a wee bit defensive! My main is very close to my heart, and I put time and effort into each component. To have those decisions criticized elicited a knee-jerk defensiveness that I had to quell before I replied to comments and made my changes.
That was my intended post on criticism, but Rhii & Analogue talk specifically about criticism in the context of raids. Rhii wants to be more conscientious about lauding improvements in addition to open discussion of problems and solutions, and Analogue wants to give tips in the most diplomatic way, so as to avoid embarrassing or offending the person spoken to. This summary seems to closely mirror my own guild's method:
When there are things the whole raid, or large parts of them, are doing wrong, for instance, not getting out of fire fast enough, then the whole raid needs to hear the correction. [...] When it’s a small change for one person, a whisper works well. A larger changeup makes Vent a good tool for discussion – but when the change only involves a single person, keeping it private can help make sure the person understands he’s not being called out or picked on.My guild has officer chat, so who knows what sort of shenanigans go on in there. We have a healing channel as well, although at times it seems that over half the raid is in it anyway. In general, I've noticed that people, especially the raid leaders, will compliment raiders on good performance. For instance, when a rogue increased her DPS significantly, it was mentioned. When we get a quick gong interrupt on Atramedes, it gets mentioned. Even when we wipe spectacularly, there are usually at least a few good points to bring up (even if it's as simple as "Good job switching quickly to adds, now let's keep doing that and also stay out of fire this time!")
As far as criticism goes, I don't know what happens in secret channels, but I know that for the most part people are quick to own up to their own mistakes, or offer suggestions to a consistent problem. Personally, when I give advice, I frame it as a question. "Could you use a cooldown at that part?" "Can we position differently so I don't get ranged from the tank so much?"
In general, people know what they are doing or have done wrong when it comes to wiping. Different from the Rate My Toon! forum, once the raid roles are established it's mostly a matter of getting used to performing all of the appropriate steps. In Rate My Toon!, people take advice, make changes, and improve. In raids, they take responsibility, apologize, and hopefully correct their mistake on the next pull. When a group is relying on you, it's important to accept advice given to you that will benefit the group. However, a group shouldn't forget that it's made up of individual members, and that each benefits from a kind word here and there. Or, as Rhii sums it up:
[...] I don’t want our raids to be all fluffy bunnies and rainbows, but balance is healthy and some positivity [is] definitely due.