Your class is your character's occupation, and you should choose it wisely. Rather than choosing a class based on what the group needs and then figuring out a way to "roleplay around it," we recommend that you choose a class you will truly enjoy roleplaying, whether in a group or on your own. If you hate playing a religious character, don't choose a Paladin, even if one is needed. If you don't like being physically weak, don't choose a mage, no matter how much your fellow DPers may be begging for one.
At the same time, if there is already a tremendous number of DP characters of a certain race and class on your server, you may want to reconsider adding yet another. Figure out why it is you'd like to play that particular class, and ask yourself if there is a way you can satisfy that desire with another class. If not, go for your preferred class... but you'd best find some way to distinguish yourself from the crowd! Often great roleplaying and/or exceptional skill with your class abilities will make you the warrior of choice even if there are a dozen DP warriors on a given server.
Another important note: In DP, there are no "useless" classes, and we don't straitjacket our players into conforming rigidly to a "formula" of how to play a class in a group. It's your responsibility to find out what the expected "default" role of your class is, but if you have a good alternate strategy or a good IC reason for fighting in an eccentric way, DP is one of the few places where you'll find acceptance and even group adventuring that caters to your preferred style. Just remember though, that if you decide to make a mage who fights with a sword instead of spells, you probably won't be the first person invited to go into a tricky dungeon.
Currently there are nine player classes to choose from, though depending upon your character's race, your selection is limited to anywhere from four to six of those nine classes. There is no "multiclassing" in World of Warcraft, at least not the online version, so choose carefully and prepare to devote yourself to your specialty. Each class has three branches of "talents" that further refine your character concept.
*considered a choice class for those expecting to "solo" often.
~prized in PvP (Player vs. Player)
^desperately sought in PvE (Player vs. Environment) groups