I have been doing a bit of reading on tanks. So, if that is the way you want to go, you can choose Gladiator or Marauder (as Lex said). Although, you are likely going to level both in the long run as they can use cross-class skills from each other that will really help both classes' tanking skills.
That aside, from what I have read on it, and from my very limited experience in pre-level 20 dungeons, Gladiator is a very good single target tank (one enemy, such as a boss) with higher defense because they carry a shield and have more abilities that increase their defenses. A Marauder on the other hand is very good at tanking multiple enemies because they have stronger attacks that affect multiple enemies that generate enmity (this is what keeps the enemies focused on you).
However, they can both tank single targets and multiple targets, just at differing effectiveness. Playing as a gladiator, it's definitely possible to keep multiple enemies from attacking anyone else, but right now I only have 1 ability to do so. It works, as long as the DPS don't jump the gun. But on single targets it is a piece of cake to hold their attention.
Also, and this might be a strong factor to consider, Marauders seem to be more FUN as they do a lot more damage and, once they become Warriors, have a mechanic called "Infuriated" where certain abilities will cause them to become more infuriated and allow them to perform powerful attacks once they reach a certain level of infuriated. Gladiator and Paladin don't really have an interesting mechanic like this. Although, managing your infuriated status might make Marauder a little harder to play as well. Anyway, that's my two cents. I hope it helps.
EDIT: I also wanted to point out that, while one reason there are not as many tanks and healers is because there are more DPS classes, it is also because tanks and healers carry the team more as well. DPS ARE important because you have to actually kill that monster you are fighting, but, if the healer or tank stop paying attention for even a couple of seconds, everyone could die. If a DPS dozes off often nothing bad will happen, unless there is some boss mechanic that requires the DPS to pull a switch or something.
). But we actually have 3 tanks and two healers so it's not needs must.
so we explained everything. Then we wiped 4 times anyway, because it just doesn't stop casting doom. I mean that pad on the floor, making everyone move around thing is quite cool but when that's all you do to avoid dying it becomes a bit silly and too difficult.
