Lighting and shadows look way better, and it follows the season, so it's all autumn colours right now. From December you get snow.

Turn it off in the plugin list and it stays off. If your PC can't run it, the client turns it off by itself.

Download it at download.world18.org (log in with Discord) and start it with World18.exe like before. The old client doesn't have HD, so grab the new one.