When I'm tanking... If I'm not in a preformed with at least the healer, I'm not going to pull more than two to three groups at a time (depending on the dungeon and size of the group, of course). If the 'someone is new' pops up, I'm not going to pull more than two groups at a time. Partly because I'm not sure of the others skills/habits, and partly because I'm not too keen on speedrunning for the sake of speedrunning. In the second case, I'm also likely to give basic tips for bosses (e.g. 'interrupt shooting sprees by doing enough damage, ignore adds while he's shooting, caster LB after he leaves' for second boss of Sastasha HM) even in a silent group.

If someone in the party has a problem with that, I'd be willing to listen to them if they speak up. If they do as happened more often than I'd like while I was still levelling and just start pulling mobs on their own, I'm not going to listen. (Though back then I was a 'good tank' and took aggro off of them and consequently died. Gotta save the squishies, right? Even when they're being jerks.)

Basically, if you're not new because you're an alt and really have done the dungeon before, it's up to you to tell me that. Otherwise I'm going to assume that at least one person is there for the first time and won't know the ins and outs of the place. =)
(And if you are new and someone else says they're not really new, that might be a good idea to pipe up :3)