Post game definitely lacks. In BW, the typical "beat elite four hur hur profit" line skews off into a battle for legendaries, and then they give you the chance to come back later and become the champion, though the E4 has stronger pokemon this time around. Not to mention legendaries, and an entire half map left to explore. It wasn't thrilling, but it took you more than two hours to do.
X and Y is lackluster so far. For starters, they just give you mewtwo. for nothing. at least in R/B/Y/G and LG/FR you had to go through a cave maze on top of beating the elite four, which was not as easy as it is in this generation since you didn't get 3 starters (event Torchic included) and early access to Exp All and Amulet Coin for minimal work. Here, it's just 3 steps in and you smack right into mewtwo.
Zygarde is a joke. Very easy to get to, fairly easy to catch like Mewtwo. Both are susceptible to Turn 1 Quick Ball, Ultra Ball, Timer Ball, and even Dusk Ball since they are both in caves.
Looker's quests are entertaining to read, but don't take very long seeing as how by the time it came around you should have had 75s. It was nice seeing guys like Jellicent which aren't available yet, and Essentia's character is very cool, but it takes about an hour at most.
Game Freak put small things like the Battle Maison (absolutely no reason why that island couldn't have been a Battle Frontier) and the Mega Stone Hunt to try and occupy your time, but in reality they get very boring very fast. There's no motivation for you to do the Battle Maison, other than trying to win BP for things like Life Orb, Focus Sash, etc.
I believe that Game Freak is doing what all those EA memes state about EA: creating an incomplete game and completing it with DLC. I feel bad for people who don't play competitively, because if I didn't I know for a fact I'd have put this game down and never looked back at it a week ago.