I disagree. It has always been the same.LFD/LFR players quality is declining, not only tanks. I just passed another alt 90 yesterday with a few friends, and trough the whole leveling we played with amazingly bad players in randoms.
I know that there always was some bad casual players who touched the game very lightly. But in since MoP it is really geting worst. From level 1 to level 90, 80% of hunters encountered had pet taunt active (not joking, we counted after nearly every hunter, and some locks, chain taunted mobs). Being at really low level, I every time asked to turn off the taunt, explaining how to do it when I had no response. Nearly everytime the hunters never cared (and were kicked). This is an emblematic exemple, but nearly every dps encountered were really really poorly played.
The fews times I wasn't tanking the tanks never could keep agro on a pack (prot paladin playing single button judgment, DK spaming ice touch, some madness were seen). Healer usually only used their instant cast (hi random rsham who let people die between two riptide... And of course not knowing earth shield @lv 80+)
And the problem is also there in heroics/LFR. With my new feral and 437ilvl I do ~40-42k dps, just near me is a ret fully T15 502 geared doing 28k... How is that even possible ? And that's just the most recent exemple. I have a blood DK, a ret paladin, an enh and a BrM I play regulary and level of play is abyssmal recently.
I really can't understand how MoP increased so much the gap between bad, decent and good players...