The wings features can be seen by clicking on the "upgrade to wings" link (or here: http://www.bookcrossing.com/plus ), and include some useful sort options, fewer ads, default journal-entry and release-note text, the ability to organize your release-zone shortcuts, the ability to choose custom "wing" styles, and more.
Some of these are more useful if you're doing a lot of registering and releasing. And some, like the "organize release zone shortcuts" one, may only be useful once in a while, if you've acquired so many shortcuts that you want to do some housekeeping. So you can choose whether you want to subscribe for a full year, or just pick up wings-for-a-month whenever you happen to need some of those features.
Even if you don't need the wings features, donations help support the site, so thanks for considering it!