What should the consistency of the meat filling of moussaka be like?
From my travels in Greece I can say that it's 100% dependent upon taste.  I've had it many different ways.  In some restaurants it's so thick that the servings sit like a block on your plate and some that the filling was quite runny and there are staunch defenders and critics of both.

As with most traditional dishes there are as many variations as there are cooks who prepare it.

The traditional recipe calls for chopped tomatoes to be mixed with the aubergine(eggplant) and meat and spices and reduced to the desired consistency.