When I first started playing the game, way back when, I didn't really pick a single class -- instead, I rolled one of nearly everything and played through a few levels here and a few levels there. I honestly don't recall how or why a warlock became my main character and my first class to 60 (and, yes, I was a warlock before warlocks were cool). It was picked, primarily, at random. I didn't have any detailed knowledge of any of the classes, but happened to enjoy leveling that one more than the rest. But after that first character, I picked classes with a bit more knowledge of what I was getting into -- after all, you can't level to 60 without learning at least a little something about the other classes. My next to 60 was a priest, whom I selected because I wanted something that was the polar opposite in playstyle of a warlock -- so I leveled a priest. (In hindsight, it wasn't so terribly unlike a warlock for soloing or questing: they're both classes with fear doing shadow damage. But in groups they were worlds apart.)
And now that I've started off with my story, I'd love to hear yours. how did you pick the class you're playing today? Was it the result of hours of meticulous research? Was it a random choice? The casual recommendation of a friend? Done because your group of friends needed tanking/CC/DPS/healing? Or did you just try everything until you found a good fit?