The impatient tanks might be partly a symptom of impatient everyone. I have tried my hand on tanking recently and apparently if I don't run out as soon as I can, some dps do and they are in the mid pull when I get there after having waited for the healer to buff *shakes head*

As for casting befire people are out of CS, it's become bit of a habit to hit the Protect as soon as I load in so I have time to get it out before people run of. Only after that I pay attention to things like rest of the group or co-healer. At that point it's easier to let the protect cast go of than to cancel it, even if I have had a co-healer with similar reflexes. And as someone pointed out, with the cast time of stone shell, there is fairly good chance people pop out of the CS right before the cast goes of.