There is so much to love about Pokemon X and Y that it'd be impossible to pick just ONE thing. So I'm going to talk about everything that I love about these games because I can. :P
Let's start by talking about the most obvious improvement - the graphics. While Pokemon has never been a particularly ugly series, per se, by the 5th generation, the 2D sprites were starting to show their age, and in a very 3D overworld, the 2D sprites didn't look very appealing. One of the things that really blew me away in Pokemon X and Y was how beautiful everything looked, being rendered in full 3D. The Pokemon models in battles looked absolutely wonderful and I found something to love about every Pokemon, even previous ones whom I didn't like very much in previous games. The graphical improvements in X/Y really breathe life into the series and make me want to play it for hours. The Kalos region is also amazingly fleshed-out and very detailed. There is so much to see and do, and it really does look beautiful. I also love the new Pokemon. Even though there aren't many that were introduced in this generation, each and every Pokemon has had thought put into it and feels complete in every way. The new multiplayer features present in the game, present in the vastly improved Player Search system, make playing with other people a breeze and really makes the world of Pokemon players feel so much smaller and better connected. I am also a huge fan of Pokemon Amie. It could do with a few more mini-games (you can only play the same 3 mini-games for so long before they get mind-numbingly boring), but it's a wonderful way to interact with your Pokemon, and I am absolutely in love with the way that all the Pokemon are fully animated in Pokemon Amie. Super Training is also a welcome addition, and it's one of the main reasons why I'm spending a lot more time on this Pokemon game than I've ever spent time on a Pokemon game before. Seeing the stats of your Pokemon increase before your very eyes based on the work you're putting in really gives you an incentive to keep training. I guess you can see from my longish post that I really enjoy X/Y. I don't think I've enjoyed a Pokemon game this much in well... ever.