For whatever it's worth, my take on it is like this: it seems like from the voter's POV, the ENnies are like a guy coming up to you and saying, "here are five good books, are you familiar with any of them?" If you are, he asks, "do like any of them?" If you like one, you "pick" that one. If you like more than one, he asks, "of those, which one do you like better?"
And if you're not familiar with any of the books of that particular group, the guy smiles and says "that's OK, although they're pretty good--you ought to think about checking them out."
And that's a perfectly valid way to do it. They aren't asking you to be familiar with every choice, and they're not even really asking what is the "best" product in a given category (because what you think is "best"--whatever that means--might not have been nominated, or it might not have even been submitted). They're just asking if, among a handful of good stuff, there's any you agree are indeed good.
(And cheers to the ENnies organizers for making the voting period longer this year!)
Edit: And I'll also add that I don't think it's any kind of judgement or strike against you if you aren't familiar with all the nominees. "All rpgs," even in today's market, is still a vast field and I'd go so far as to say there's no reason even the most avid fan should know the ins and outs of every game out there. I certainly wasn't familiar with every nominee either. I think I had to leave at least one whole category blank, although now I don't remember for sure.