Maybe I am just irrational, but I really hate people who exclaim about the ease of whatever encounter it is we happen to be about to do. Even if we out-gear the instance and are likely to find it easy. There's nothing that causes wipes more than arrogance.
When I was in my first real raiding guild we did a Righteous Orb run through Strat Living. Now, we'd downed Raggy and Hakkar that week and the guys were talking as if we were immortal. It was a full guild run and they were all laughing and joking about how we should make it at least a challenge and have me (mage) tank.
We wiped on the first lot of trash. We wiped hard.
The warrior said he'd start to use a one-hand and shield. We wiped again.
The healer said he'd stop trying to DPS, we agreed that would be very helpful.
Eventually, after a few more wipes and a lot more humility we managed to finsih the instance.
What I'm trying to say is, instances are easy so long as you respect them and accept that you still need to try.
When you have a mage (as I just had in my run) who seems morally opposed to the idea or re-sheeping, who boasts about how he needs to cut back on the damage because his DPS is so rocking (no, mate, you're pulling agro because you're starting with Pyro on a mob I've just pulled with Faerie Fire), who tells the other members of the run who've never been in before that Murmur is really easy, and who seems totally unable to follow the simple kill order (this is another reason you're pulling agro, you aren't hitting the target I'm actually tanking). When you have this mage in your group, you know it's going to go badly. You can't trust him for a start, because he wont re-sheep. You can't keep everyone focused because he's constantly bragging about how easy it all is. You can't keep the healer alive because you're all over the place trying to tank mobs which should be CCed, which the mage has activated and then Ice Blocked to get away from.
I get annoyed at players who make things harder through their arrogance. I try to approach each instance as if it could kill me, which it almost certainly can if I don't treat it with respect. I find this works well enough, as the tank can have a certain calming influence over the rest of the group. However, some days you're fighting a losing battle and the best you can do is to suck it up and keep swiping.