What if anything make a liquid a good candidate for a reduction? What if anything makes a liquid a good candidate for a reduction?  Often you see things like wines and fruit juices called to be reduced for recipes.  Why are they so common?  What specifics about them make them so?  

If I am wanting to play around with making different reductions is there anything I should look for and what are the expected results of the liquid having that content (i.e.  high sugar content, does acidity levels have an effect on the reduction).
The most important reason reductions exist, is because they intensify the flavour, so that's the first point you should pay attention to. Will the flavour accommodate the rest of the dish? White wine is often used in reductions for fish, red wine more often for meat. Everything that you think will fit, can be a good fit (other (stronger) liquors, vinegars (but don't use too much), ...). That's why you won't see liquids as coffee appearing in reduction recipes.

Wines and fruit juices give a fruity, softer taste, so they are a good fit with a lot of things. They also are largely water, so easy to combine with.

I must confess that I don't know how sugar content or acidity level will effect the reduction, but I do know that sugar and vinegar/lemon juice can be a component of reductions.