Tactful ways to inform people that they're playing badly?

[Raldo]

Avoid using a non sequitur as a rebuttal. No one would want to engage with you if you don't engage with them.

Congrats, you finally made me look up what non-sequitur means.

[TheMightyMollusk]

Shirk them and watch the fun

It's impossible to find a way to say something that somebody can't get offended over, because some people just live to be offended. Just be as reasonable as you can, and if that's not enough, vote kick 'em or leave. There's no point trying to fight the online disinhibition effect, so all you can really do is remove yourself from the situation.

[Rasikko]

Just remember that command type phrases tend to put people off right away.

[Fynlar]

Can't be done. Phrase it however you want, they will still either remain silent and completely ignore whatever you suggest (happens 99% of the time) or bust out some sort of offended retort, and they will have fuel to use against you if they are feeling particularly petty and call a GM afterward. This is why the best thing to do if it's really that bad is to just kick them without a word.

[Berteaux_Braumegain]

Usually, I try to ask an entry question so there's an exchange of dialogue.

For example, I had a Level 30 Marauder in Brayflox that also had some missing gear slots. I asked them "(Name), did you finish your Job Quest?", at which point they realized they didn't have their Soul Crystal equipped and couldn't equip it. I mentioned that it was alright and just something to be mindful of, and then pointed out that they could also get some accessories to bolster their HP in the hub cities (I avoided directly stating that their health was lower than mine).

They seemed pretty receptive of my advice and no one else in the party jumped down my throat about it, so I consider that a win.

[Saintly]

Just vote-kick them and pray that you don't have any SJW/white knight in party that will vote no or worse make a fuss about it.
Anything else will either be a waste of your breath or earn you a warning/ban from the trigger-happy GM's.

Other than that we can just wait for SE to realize how silly and pointless their "don't talk about parsers" rule is, so we can state that someone doing bad in party and back up by solid data.

[Berteaux_Braumegain]

I just type what they’re supposed to do

Yeah, usually if I see someone messing up on mechanics, I tell them to stick close to me (assuming there's no mechanics where you don't want to be close).

[legione]

bad and lazy players are what keep levelling content entertaining ,i dont get why you want to get rid of em.

bb

[SturmChurro]

Whenever I play I usually just move to sarcasm, but you need to pick your words carefully otherwise you could be breaking the ToS. Trying to inform a player that they are bad is probably the hardest problem I have faced in this game. Normally in any other game I would just say a certain word a certain combination of words to address the bad player(s), unfortunately, like I said, you would be breaking the ToS, reported, and subsequently banned. I won't even say what word(s) I would use here, because I would also be banned on the forums.

This is something a deal with on the daily whenever I am playing this game. Usually pointing out their gear, inform them they aren't doing aoe, or saying "our dps must be low" - speaking in general should inform player(s) that something is not right.

[PyurBlue]

Make a simple suggestion when you can. If you're acknowledged then it payed off, if not it's not a big deal. A few weeks ago I ran into a party of sprouts where the two DPS were very clearly not aware of what was optimal or just not eager to optimize at all. One used the same attack over and over while the other one would never AoE. I said something like "Hey player A you will deal more damage with combos, and player B using AoE's on big packs will do more damage than single attacks." The first DPS ignored me, the second one thanked me and made an effort to use AoE from that point on. That's just how it goes.