With the direction the conversation has taken I think it would be good to summarize a few points from what I see as the loose grouping of people on 'my side' as it were. It's by no mean me claiming unilateral endorsement of these ideas from everyone who favors advice and improvement, if I'm misrepresenting anything please call it out, but I'm seeing the same few objections from detractors.
Primarily I see the frequency with which any one player might encounter play poor enough to warrant advice brought up; the "don't let one bad experience color your thoughts on players as a whole" argument. I'd say this misrepresents my own experience at the very least and I see it said specifically in response to anyone bringing up a specific story of a time someone underperformed.
Instances of someone performing badly enough for me to offer advice happen maybe once every day or two, outside of EX+ content. These are usually the gaping holes in competence, things like cure spammers who do no dps and truly freestyle dps or tanks who seem to just be pressing buttons randomly or with no care. My Shield Lob PLD in Copied Factory comes to mind.
To be clear there are many, many more times people are also doing just awfully but there just isn't a chance to actually try to help. The number of times as a tank or healer that I end trial or normal raid roulette above some dps who haven't died... it's really frequent and it shouldn't be. Those instances are too short to generally get out much in the way of help though.
Secondly I see the "rudeness" argument, primarily championed by Goji. While I don't think anything anyone could say would get even close to changing his mind (and clearly people have tried) I would say that intent matters and that it's being entirely dismissed here.
If you speak to someone with the desire to cause them distress and embarrassment that's bad. If you speak to someone with the genuine desire to help and someone gets embarrassed as a consequence you need to weigh whether or not the advice you gave was worth it. I'm not going to nitpick every little bit of someone's play in casual content, if a tank wants to pop CDs as he finishes a big pull rather than waiting for some Holy stun mitigation that isn't the end of the world. If DPS aren't banking resources to destroy big trash pulls and instead popping it all on bosses that's fine, we'll still be moving apace.
The issue needs to be great enough for me to think the potential embarrassment or annoyance (or even maybe being kicked on my part, you never know who's in an oversensitive premade) is worth it on the off chance that the advice penetrates and helps that person do better in the future. This is delicate but I think I'm getting the hang of it.
That kind of touches on the last point that I've seen brought up less frequently but still pops up now and then - the actual standard to be held to in order for advice to make sense to me.
For anyone if there ignoring a big part of their kit I think it's worth pointing out. You never know what button might not have made it onto a hot bar.
For tanks they should be able to hold threat 100% barring a dps getting dumb and bursting on a single mob during a big pull or something. They should be using all their CDs potebtially aside from their immunity because I know getting good use of LD/Holm during dungeons can be weird. AoE on trash, use ST combos on bosses, pull big.
For healers they should be using their ogcd heals frequently and filling in the gaps with the appropriate ST/AoE nuke. Spamming heals on people with full hp or something is bad. They also dont get to stand around doing nothing. I realize sometimes they do this because of MP, those same healers usually have Lucid ready and waiting until they're at like 2k MP. I think I tell more healers about lucid than I do about doing dps.
For dps it can vary but it boils down to AoEing when appropriate and doing enough of what would be their single target rotation to at least keep pace with the tank/healer. If I'm on a support role I should not outdps you, period. Standing in too much bad or, as ranged, being out in the middle of nowhere and thus out of range for heals is also notable.
So there we go. If you think those standards are too high have at it. I am about as immovable as Goji is on this. Clearly my standards for content with enrages are very different but if I meet someone in casual content I'd expect them to be able to do this much and if they can't (and if I'm able to) I'm going to give advice. It isn't rude or toxic, it's trying to get people to be better for both their sake and the sanity of their future parties.