A couple of posts in this thread are fatuous, and are nothing more than posters going full-reactionary mode and forgetting that this is a demo. There's no doubt that I'll heavily criticise EA when the final product is released - FIFA 16 was terrible - but it takes more than one day to judge something. Also, something interesting (not really) but many people loved the FIFA 16 demo and overrated the hell out of it. And look how that turned out.
Anyway, my thoughts:
Passing has improved - not by much, but to the point where you can notice a difference. In FIFA 16, when you play a through ball, it goes straight to the defender. Which always irked me. It's as though EA forgot that through balls are played into space. It's so much better on the demo because you can pass the ball into space, with your players running in behind.
Sliding tackles seem to be nerfed. Don't know if that's a good decision, but they had to make the change into a more "manual" system because it's ridiculous how easy it is to tackle an opponent on FIFA 16... unless they're really skillful. LT/L2 seems to be an all in one button for defending, which is great, and defending seems to be more about skill. Not sure how I feel about the whole concept, but it's something different.
Players feel heavy and I feel that makes it more realistic. Let's be honest here, EA copied PES in that regard. It means that you can't spam dribble and hope your player turns right/left quickly. You can still run in a straight line and pace abuse to a certain extent, but it's trickier on this game. However, the input lag on this demo is horrible, and there's no doubt that it's adding to the overall sluggishness of the game (along with the frame rate dropping like crazy). Really, really disappointing.
AI is still too good. PES has the exact same issue as well. Impeccable ball retention, which really bores me. I can't remember the last time I played career mode religiously (maybe FIFA 09 with Leeds?), so it doesn't really bother me that much, but EA need to understand that lots of players avoid online and solely play career mode, plus the demo should give new FIFA players a better experience IMO.
Finally, I'm on the fence in regard to the new penalty system. It seems convoluted this year. Honestly, is there any need for it? It was fun initially but I'm not sure if it's a good idea.