I recently heard someone talk about how we have really had six world wars, not two. According to this person, the thirty years war, the war of the Spanish succession, the seven years war, and the Napoleonic wars should all be considered world wars. Is there any merit to this idea? By what criteria were these wars similar to the world wars, and in what ways weren't they?
I can try to answer your question in regards to the Seven Years' War.
The Seven Years' War was a "world war" in the sense that it was a conflict that had related battles fought across the entirety of the war. This war had battles fought in Europe, North America, South America, India, and West Africa. So if the definition of a world war is taken literally, then yes, it would be considered a world war. I know less of the other conflicts listed, but I would not be surprised if they fell into the same category based on the global nature of the conflicts.
The difference between these wars you listed and what are commonly referred to as the two World Wars, however, is that World War I and II were examples of total war, that is they were wars were the entire country was almost directly working towards winning the war. I've heard arguments made that the Napoleonic wars were also total wars, so if the definition of World War is changed to "a war that involves total war and global battles" then one could argue for three (Napolean, WW1, WW2), but arguing for the other wars listed it would be a bit of a stretch.
No, because that's not what we call them, and it's really that simple. There is no good reason to try and shoehorn some definition in here -- it always breaks down. I mean, was either of the Iraq Wars "world wars"? They had global participation between some of the top military parties (Iraq was #3 in military size in 2003, US #1, etc. for the first one too), had global participation (America's, Middle East, Europe, Asia), and so on. Trying to define "world war" is always pretty arbitrary.
It's just as simple as that is what we call WW1 and WW2 so that's it. WW1 wasn't really called that at the time, only when ww2 happened and their connections made did those names follow. They were given special names by society as society deemed them uniquely cataclysmic and globally inclusive wars to call it such. There is no cabal of historians who name wars, society does that more or less in this case. So the thirty years war is called such because for wars in that period, we tend as society to refer to them by their lengths (100 years war, 80 years war, 30 years war, 7 years war, etc.). We call the Napoleonic Wars that because of their connection to the man Napoleon and his dictatorship, etc.