Lately, it seems like there have been more tanks who just have no idea how to tanks mobs. They'll just attack a single target and leave all the rest to go after the rest of the group (namely the healer). The tank doesn't need to grab *all* of the weaker enemies because the dps should kill them in short order, but you can't let multiple strongs and elites go after the healer.
I had an HM Cademimu run on my Sentinel the other week where the tank (who was in ~50% dps gear) never attacked more than one target at a time. The healer was geared well enough to keep him alive despite his lack of tanking gear, but had a much harder time keeping himself
alive when all but one of the mobs went straight for him. I ended up spamming Twin Saber Throw and Force Sweep to try to get the mobs off the healer in every pull. I basically ended up tanking 90% of the mobs in the instance while the tank and other dps just went through their (seemingly random) target priorities. Fortunately, between the healer's gear and my defensive abilities (would not have wanted to try that on, say, a sage or operative), I was able to survive playing the "sentinel tank".
And here everyone complains about how sentinels are convinced they are actually tanks.