Considering water as a given, the third most important ingredient in a copper electroplating solution, as a short survey of related recipe webpages will show you, is hydrochloric acid (HCl). Here's a sample link: http://www.thinktink.com/faqs/cupltfaq/cupltf01.htm
I can guess that the role of sulfuric acid (the second most important ingredient; the first being copper sulfate) is to provide you with sufficient anions to allow copper cations to easily enter the solution at the anode. But in almost all recipes that I have seen, a small amount of hydrochloric acid is added. I wanted to know how useful it is and what is the opportunity cost associated with it.