Being fully 3D, as it is, is a huge selling point for X/Y. People like me (25 yrs old) have been dreaming of a full 3D Pokemon RPG for years. The graphics and the similarities between the last three portable versions have just led me to become very uninterested in the series. This will be my first Pokemon game purchase since Ruby/Sapphire, and that kind of interest from long-absent fans like myself will probably propel X/Y to be some of the best-selling Pokemon games in a good many years. Nintendo is back!
I think the only factor that might hold X/Y back from breaking 16-18 million in sales is the softness of the 3DS market in the west. Europe especially is just not doing well right now. International publishers over there are scaling back, laying European subsidiaries off, limiting the number of releases, and even taking it to the level of only releasing digitally so they don't risk taking a bath on disappointing packaged-software sales.
If Nintendo and Pokemon Company manage to make X/Y a massive success in Europe as it will be in Japan and North America, it could see sales near or exceeding 18 million. That would truly be something to behold.
