I agree that acoustic pianos keys should have a bit of a bounce as there is weight, so the lower you press down, the more force on the bounce up (I think that's physics but not too knowledgeable with that). A salesman told me the Casio PX150 has 95% weight, the Yamaha PX115 has 90% weight or something like that. I think it's more of an issue if the key doesn't come back up fast as you can't play faster than the rate the key returns to original position. But the sound of an acoustic piano is loud enough to cover off the mechanic sound of the key action mechanism; the instrument is designed after all to amplify the sounds of the strings. I used to use the middle damper pedal to muffle the sound so I can play at night on my upright without disturbing the neighbors. I purchased the PX-160 recently and I did notice the key noise a bit when I attempted to play Mozart's Turkish March, since I have to play in both softer (piano) and louder dynamics (forte) fast I think it's just the mechanics, like how normal computer keyboards make sound when you type because it's hitting something below that's different from what's hit on a real piano, a string. But I don't think it's normal if there's a lateral movement; I have no idea what that means so the keys move from side to side? I would say that would be a concern if that happens. It's normal if the key bounces vertically but laterally? That's seems like the key is loose.
