My tank joined a PUG for Random Veteran Dungeon last night and after the first boss i said to the group "DPS seems low".

One of them agreed and one of them apologised and one of them thought we were doomed and should quit.

I told them "We can do it, it's a matter of will", knowing that my tank would be okay at least.

Due to the low DPS, the bosses took ages but we all persevered, got into a rhythm and completed.

It helped that i can type/rez and tank at the same time, but communication is key.

If the DPS seems low, say so, just like that, no need to be confrontational about it.

Depends on what do you mean by "sucks". For one of my friends everything below 40k/dummy sucks and i usually shut him up when he starts to annoy party members with 25k about their dps.
But theres another number - 15k (10k on low lvl). It is possible in any gear without skill with a little effort (i.e. even spamming HA + Wrecking on 2H) and it is enough to do any non-DLC 4-ppl pug content. So if your party member does 5k or less he simply doesnt give a damn about you and the whole party probably considering you all morons and is making no effort at all, maybe spamming light attacks or AFKing mostly. In this case feel free to tell him who he really is.

"Sorry, but your DPS/healing/tanking is not sufficient to complete this run IMO. If you can improve now with focusing more, or you don't understand the necessary mechanics, just say that, we'll figure something out. Otherwise, sorry, but we are stuck on this boss and I don't see this group completing it."

You can state something without being rude. Some people need to accept that certain dungeons and trials require better performance than what they can currently produce. And you can always help each other by explaining the mechanics or helping someone understand their character better.

1. If the OP wants to see examples of truly, spectacularly, fantastically awful individual effort in PUGs, albeit PUG PVP, they ought to give World of Tanks a whirl, preferably on the non-NA servers for a larger player base.

To be sure, 80%-90% of WoT players either know what they are doing, know what they are supposed to be doing, or are stably and predictably average-to-mediocre and thus not actually detrimental to the team as a whole most of the time. The rest of them, however...Leeroy Jenkins weeps in shame at their antics, let's just leave it at that.

...My point is that by comparison ESO PUGs (or TOR, or anything comparable) are relatively tame. On occasion one gets the odd crop of infantile idiots or simply bad players, but then the content difficulty level is, trials excepted, not such that this cannot be overcome in a majority of cases.

2. The principle I prefer to apply to PUG play, having done a whole lot of random-team-deathmatches (and I mean a whole lot, probably well over 20k just in the WoT/WoWP/WoWS triad), is that it does not matter who does what so long as the team wins. So long as you win, it does not matter who stole whose frag, who had to carry whom, who acted like a complete moron, who has an unbelievably bad rotation and gear setup, and, where applicable, what kind of victory was scored (e.g. frags vs. capture points). A win is a win is a win. To use a sports analogy, it does not matter whether you win by a single point or by a hundred (excepting obviously tournaments or qualifiers built explicitly on goal differential).

Obviously it makes for a lot smoother experience if everyone in the group is above-average, knows their role, knows the content, et cetera. But insofar as an ESO PUG run, so long as the boss is down and the loot collected...the rest does not really matter. That's the break point for me, which, again, given the not-very-high difficulty of a lot of PUG content in ESO means that literally 90% of the time I try to do my job and generally do not care whether the rest of the group knows what they are doing since the bosses are getting burned down anyhow. And the other 10% of the time my only concern is to make sure some boss mechanic is respected as something required to burn down said boss.

The only exception to the above would be something like a trial, which, given the elevated difficulty level, would mean that certain DPS minimums need to be met, et cetera. To compare with the three raids in Marvel Heroes - at least they only had the three before I left - anybody could do the first one (green Musp) because it was so easy; some gearing and preparation was required for the second one (red Musp), which was rather more involved so the lead would typically inspect gear or look out for mechanics mistakes; and the third one (Onslaught) is where inspection was mandatory, people with inadequate gear would often just get kicked without any hesitation whatsoever, and some leads would literally time you on a DPS dummy before letting you in. So yes, at an ESO trial, I'd expect that sort of scrutiny. For a PUG dungeon, however, especially on normal? Tcha!

3. Just as a potentially bubble-bursting aside. For one, a majority of people in general are "average" (i.e. if one accepts a normal distribution of particular traits in the human population, 2/3 of said population will be situated within one standard deviation from the mean). But specifically in gaming, if nothing else, WoT player stats (across a very, very large sample) show that the overwhelming majority of online gamers are bad-to-mediocre-to-average-to-slightly-above-average. The casuals, the kiddies, the working stiffs, the "I only play while drinking" crowd (that one's my personal favorite, for obvious reasons). In any MMO with a sufficiently large player base, that is the PUG environment, statistically speaking, even more so in an MMO that happens to have some complex or non-obvious mechanics (e.g. bar swapping).

Again, my personal approach is not to care so long as I get my "win".

I wouldn't go the "you suck" route. That's not helpful at all. I would take a less personal approach by telling them their dps is too low. Afterwards, offer to help them with their build and rotation.

I was tanking BC2 last night. This healer had the nerve to tell the stamina DPS "if he ever saw them in another dungeon, he would immediately leave." Rapid regen and mystic orb was the extent of his support.

Mind you, the dps were doing fine. We managed to complete BC2 on HM. They were extremely cooperative. If i told them to jump, they'd ask "how high."

Point is: there really wasn't a need for his abrasive comment. He could have simply offered to help them. And they would have been more than willing to listen to his advice.