Here are a couple of basic features (I'll leave it to less biased users to sell them): You have three symbols under your name on the upper left: a hubwheel, a badge, and a follower icon, with counts to the right of them.
The hubwheel goes around and completes as you use the site, and others appreciate and share your contributions. Every time you complete a hubwheel, you earn a badge that you can award to another user's post or comment. Click 'badges' in the menu on the left to see what has recently been badged by others. The center of the hubwheel turns orange if you have non-dismissed notifications. (once you earn 1 hubwheel, you can also start adding community tags to posts)
The badge icon keeps track of how many badges others have given you.
The follower icon is pretty self-explanatory.
Click the 'markup' link on the top right of a textbox to see how to do text functions like shout outs to other users, like this one to b_b. In general markup requires book-ending text with symbols.
Under 'controls', you can switch your hubski style. In addition to dark, you can choose 'snow', 'clean', or 'ugly'.
You can also post images in comments.