I can give a quick rundown of some features, others are dependent on other features, so it really takes a bit of exploration on the settings screen to customize NG to your liking, but some of the more major ones are:
Collapsing the Quick Links at the top of the page to only the "Social" elements, such as Friends, Messages, and Events. The other quicklinks become a dropdown menu under your username.
Suggesting your friends' names when you begin typing in a PM. Newgrounds has since implemented their own version of this feature, however, NG+'s includes profile pictures of the users.
Adding the "Respond" button back to the forums.
Adding an inline poster, so you never have to leave the thread to post.
Showing what percentage of your post is quoted.
Hotkeys for New Thread, Quote, and Respond.
Automatically collapsing BBS images.
Adding an ignore list to hide posts from certain users.
Autoexpanding the review box for movies and games.
Auto turn off the lights and widescreen movies/games.
There are a bunch more features, but like I said, without the settings page in front of you, it gets kind of difficult to explain what everything does.