Edit **So after rereading this post it comes off more like a rant. The short of it is; I, and I'm sure many others, don't find tanks all that fun to play at the lower levels. I believe that's due to the lack of visual feedback the classes have. They don't show you anything to feel really good about until much later into the game. I say this as one of the people who could be tanking but isn't. I am part of the reason why there are so few tanks because I'm not doing it. Playing a tank just isn't as satisfying to me as most of the other classes are. And I'm sure I'm not the only one who feels this way.**
Okay so, here's where I stand on tanking. I don't have a 50 tank. GLD is 15, and MRD is 35 I think. Haven't touched it since before 2.1. I stopped tanking long after I got bored of doing it. Is tanking more fun at 50? I want to say yes but I obviously don't have that personal experience for a definitive answer.
But I had fun leveling my WHM and MNK from level 1 - 50. Something about them was really fun. I like LNC, ARC, and THM a lot too. They do damage, and good amounts of it. They do damage that I can see. I'm playing a damage dealer, and I see damage. I don't play my gladiator, my marauder, or my arcanist at all. Gladiator and arcanist are 15 for job requirements and I don't plan to go further anytime soon. And I first started wanting to play warrior so that's the only reason why Marauder is up to 35. I don't like arcanist so much because I can't see my damage like I do the other damage dealers. It's there and it's certainly good damage. But it's somewhat dissatisfying. DoT's are all rolled together into one barely visible number every 3 seconds, and my carbuncle's damage isn't any easier to see. So what DO I see? Ruin. Oh boy.
That's how I feel about the tanks. I'm sure that's how many feel about the tanks. Truth is low levels are the most important because they need to get you into playing that class and the game as a whole. That drive to keep leveling is just not there for me in tanking classes or the arcanist. You can't see what tanks do at the lower levels. At those lower levels it's like playing a DPS class that does crap DPS(okay maybe not MRD). You've got tanking cooldowns but the low level content is so easy you can get to 50 without using them once, or at least to level 40. Hell you don't even really start to need a tank until around level 25. I mean come on, the healer can fricken tank with just cure aggro, I've done it in Toto Rak and Titan normal mode. Good the tank class I chose is actually not needed and frankly slowing my group down, another LNC could easily take my place and nobody would really even care.
I'm sure mitigating mountain buster or death sentence is pretty frikken sweet to see. Maybe it's the most satisfying feeling in the world watching king behemoths meteor drop right on top of you while you /laugh and still be standing when the dust clears. Where is the satisfaction levels 1-49 then? My issue with tanks is even if end game tanking is fun as hell, and it looks fun honestly. I don't want to be bored getting to it. Instead of having fun leveling, I'm being led on by the carrot on a stick that is the endgame and promises of fun. Getting gladiator to even 15 felt like a chore
That, in my opinion, is why we have such a large lack of tanks. There's probably a lot more like me that don't have fun tanking right now because it doesn't show you in big flashing numbers with an exclamation point how awesome a job you're doing tanking. Simple solution to me would be to buff tank damage, like you said earlier, in a way that requires them to be tanking do so, so they can't replace real DPS. It doesn't show you how well you're tanking sure, but it is something that can be seen and that many people like to see. I'd definitely play the hell out of my tank then. Until that day though, likely the last classes I'll max. And the disparity between tanks and other classes probably won't get much better.