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

[Raldo]

If someone isn't using the right abilities, I'd have no idea, since I only play NIN, and even then, I don't know how to tell what abilities another player is using anyway.

I level everything to cap. I won't consider myself an expert on them all, but I at least know the basics. Some jobs are difficult to analyze, but if a dungeon run feels slow, that's when I start looking at peoples' buff bars, cast bars, enmity list, and occasionally battle animations to see who isn't pulling their weight.

You can tell if someone isn't AOEing at all if they have zero hate on some stuff during a large dungeon pull, for example. SMN has a lot of tells: DoTs dropping for extended periods, no pet, never getting Further Ruin buff. Several jobs have a damage+ buff to upkeep, so you can look for that. AOE pulls w/ BLM, but you never see the giant Flare animation. etc.

Tanking is often boring, so sometimes I analyze what others are doing to occupy my mind.

The OP asked for help here, not there.

I replied to you in that thread, so can you please contain your irrelevant responses over there. The thread has long since moved on, however, so I am not looking for replies anymore.