As a tank (WAR 50) I know my limits, and I always gear check the healer (and occasionally dps) to have a rough estimate of their limits. So I don't find anything wrong with giving instructions to the DPS as to what they should and shouldn't be doing. I am choosing a class that stands there and takes the hits for everyone, and allows them to complete the instance.
Tanks are always the hardest to find (esp. in DF), and not to sound elitist, but sometimes I enter instances as a tank just to do the service of helping others through it, so I think it is fair if I take charge. Letting the DPS know what mobs to target, and in some instances what skills to use (and not use - I'm looking at you dragoons in Ifrit :p). Also things like asking a BLM to sleep a group, which allows both DPS and healer to burn single targets (the time it takes to sleep is easily offset by the damage gain from the healer).
