Lack of tanks is not an issue about how "fun" the role is, but the responsibility behind it. Tanks are generally your leaders. The group typically moves at the pace of these players. No one wants to lead a group, they're just more comfortable following... and subsequently complaining lol. If tanking did not involve the pace mentality and responsibility of preventing your target(s) from breaking off you, you'd probably see more people take up the role.

Tanking, to me, isn't very far from PvP. What I mean by that, is that in PvP, your targets will attack you, which for the most part isn't going to be an issue if you can be healed through it or you get other means of help to win. Likewise, with tanking, you're not going to have an issue if your group can keep you alive long enough to finish a fight. So if groups were designed to not necessarily have a "tank" role, but that people were to just be healed through damage while they DPS away, I don't think anyone would mind "tanking" fights. However, they would still mind the responsibility of being leader, which is pretty much expected of the tank role (as a default).