Another problem is sub-geared tanks. DPS queues as tank with no real tank gear, they wipe then quit. If you got one of those tanks as your solo tank, you'd definitely fail. The biggest reason I don't queue as a tank is that people are brutal to tanks in how they treat them. I think pretty much every wipe I've been on Dark Shaman is because of "noob tanks" according to the players in LFR.