RDM is the mash up Jack-of-all-Trades, many jobs can primarily be seen as a simple combo of two of it's aspects.

RDM's meleeing + RDM's buffing = BLU
RDM's meleeing + RDM's nuking = DRK
RDM's meleeing + RDM's tanking = RUN
RDM's healing + RDM's tanking = PLD
RDM's healing + RDM's nuking = SCH
RDM's buffing + RDM's enfeebling = GEO

I admit it's a rather simple way of looking at things, but I think it's fairly accurate. The three brought up right now are BLU, GEO, and RUN, each of which admittedly are viable alternatives to RDM provided you're ok with limiting yourself to such roles. If you primarily like RDM's meleeing power you're likely to find BLU and RUN interesting as they focus on that sort of melee power as well as one other massive aspect, and a few minor ones. For instance, a BLU can enfeeble or a RUN can buff, they just don't do it very well. I personally like RDM more than any of these alternatives.