When I started lifting weights regularly in 1996, the standard practice in the gym was to put your weights away. While some people didn't, it was the exception rather than the rule, and most people took care to put their dumbbells away or unload their own plates off machines or barbells when they were done.
Now as I look around the gyms I see plates and dumbbells piled everywhere. While some people are still obviously taking their weights off, many do not. This shows a general lack of courtesy and concern towards your fellow gym goers. If you've got the strength to load up a barbell or machine with plates, you should be able to take them off. Same for dumbbells.
I'm sorry to say that I also see trainers not puting away weights after their clients or themselves. We've got to set a better example during our time in the gym.
So, everybody, treat the gym with respect, be courteous towards others, and don't be a douchebag -- put away your own weights.