It's hard to say what classes aren't played much. It will change depending on who you talk to and through the levels. Are you concerned with raiding or casual play? Do you want to be in the action or on the side-lines medding? Your independent goals will shape what class is best suited for you.
I have noticed a very small amount of Bards. I chose a Wiz after talking with a few people from what seem to be the raiding guilds on the server and asking for feedback. Wiz was one class that almost everyone I talked to said was in short supply (probably because they are rougher to level at low levels than other classes I've played, just because we're extra-squishy). I also rolled a Bard so that when I get irritated with the Wiz I can switch over to something different for a bit, but I think I'll stick with the Wiz.
It's also hard because there may be classes, like Druids, that are played only to the mid-level range. If you haven't played in a while then I suggest rolling a few classes and seeing which one really grabs your attention. It really comes down to what you enjoy because if you end up playing a class you don't really enjoy, you won't be motivated to play. And also remember that the first 10 levels or so will be filled with death....and accomplishment.
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