Well, the match-making system does try to "spread the love" in terms of class/role/gear representation (as well as CC abilities). You can get the groups with doubles of certain classes, such as a Feral and Guardian in the same group... but it's usually a premade group if that happens. Also, since there are generally more DPS queuing than healers and tanks, sometimes you might get the same classes since the pool of tanks and/or healers is small.
I've seen a pretty even spread of all tanks in random heroics. In LFR, I personally do not see druid tanks that often... usually paladin/DK on my end, but that's just me and not an across-the-board fact. I will say, though, that the druid tanks I have come across really aren't that great, and I try to help them out when they take constructive criticism. The only tanks that I honestly haven't seen change much since 5.0 are DKs, but that's to be expected since they didn't change much at all.
Also, as a druid tank myself, I really only queue when I can get the Satchels. Otherwise, I'm learning my alts and figuring them out... takes a while to get through every class and master it.