Bathroom above the Garage
List of issues:


Getting the space conditioned.  This may be really easy or insanely difficult.  Depends on what is on the other walls of bathroom and where your ducts are in your house.  
Getting electric.  Should be no big deal.
Getting water.  Might be a bigger deal than electric but probably not a huge thing.
Getting exit plumbing.  Given that the bathroom is on a second level you have a lot more options but you will probably have to open a few walls to get plumbing to your main stack.  Your shower and faucet aren't a big deal.  It is your toilet that would be concern.  You don't want to run a long solid waste line with low slope.
Venting.  Should be easy since you are on second level.


So biggest issues are #1 and #4 for a bathroom above garage.  If you get these easily figured out the rest shouldn't be a huge deal.