RDM is definitely relatively simple to play, but it's something you sort of have to get into playing hands on. It's a very rhythmic class so just reading a rotation guide may not really be as helpful as just going out and hitting something. I'd actually suggest going to Palace of the Dead and doing the first 20 or 30 floors so you can see how the class is "built." I can absolutely see why moving to RDM (or any caster, really) when you're used to melee would be an awkward transition.

It's also worth keeping in mind that while RDM is simple, it can come with some expectations and responsibility. If things start going south in a fight, you're basically expected to jump in and do everything you can to right the ship. So this can mean going from 100% DPS mode mentally instantly to "Oh crap, both healers are down, the tank needs a heal after a tank buster and only I can fix this" mode.