Just been watching Homes Under The Hammer and they often do get houses with bathrooms downstairs and seem to think its a bad thing. Ours is downstairs and even though its a bit odd I kind of like it. Its much easier to bath the kids and dress them in the front room. Also if I fancy a bath on a night or when I get up before everyone for a shower I am less likely to disturb anyone sleeping.
So whats wrong with your bathroom being downstairs?