This really resonated for me. I've often thought through 'viral' aspects of possible products, but they've always been 'A YouTube video' or 'A themed game' kind

This really resonated for me. I've often thought through 'viral' aspects of possible products, but they've always been 'A YouTube video' or 'A themed game' kind of thing - relevant to the brand, but light enough for people to pass it along. I've called that viral, but it's not (any more than a Facebook meme, which I can ignore, could be considered more viral than smallpox). Viral is Hotmail (and the other examples the OP lists) - when a person uses the actual product, they cannot help but spread the message ("Sent via Hotmail. Register now for your free account"). Great observation that I had previously missed.