Well... Can't exactly say much for the SMN/WHM thing, but for me, in every MMO I've never taken the "tank" class (always thought it was boring standing there swinging a sword every now and then). So I started off being a thief cause I wanted to go RNG. After realizing I did NOT like playing a thief and there's no way I'm gonna hit lvl 30 as is, I fooled around with a BLM. Well, spells etc is fine... But then I missed actually being able to take a hit (mob *attack* POW! I'm dead). So what do I land myself on? Something I thought I was hate! The warrior class. Turns out, wow, I love it! Even though I disliked tank classes in almost every other MMO, this one was surprisingly fun. I realized for this game, I didn't want to be the guy that does the most damage, or the person that can stand in a corner and nuke. I wanted to be a character that would help others in my team, thus I'll eventually go the PAL/WAR track, and become a shield for my team
So basically, even though you've been a mage in every other MMO, give the other classes a shot. Lvls 1-5 can be attained in under a couple of hours w/ no sweat.