So this time there will be no internet spoilers before your adventure.
>tfw you won't know most of the types of new pokemon
>unknown legendaries (besides Xerneas and Yveltal)
>most of gym leaders and their gyms
>completely new storyline
>new abilities, attacks, items, etc
>elite four and champion implies more of a challenge
>starters and their evos
>other's evos
>new evolving methods
>possibly new evos/pre evos for older pokemon
>game after completion

I'm sort of excited that Pokemon X Y will be released the same day worldwide so we'll get a different experience than previous games. (though, if you want to be informed by the internet in the first days, it's your choice!)