For me the things I really enjoy about WikiTree are not that hidden. But just the things that make it great and easy to use.
For example, just the basic profile creator, it is easy to use and fill in all the boxes and edit information later. On the other genealogy websites I use like Geni, FamilySearch and Ancestry I feel like it is always more of a challenge to enter in the data and have it come out looking like you want it to. At least you always have to click a bunch of icons or boxes and wait for things to refresh. So I enjoy the editable wiki approach to genealogy profiles.
Also I really enjoy the G2G forum and the ability to "Ask a question" tied directly to a profile. This is a great help as there are usually other skilled genealogist willing to help look up information on a profile I may be having difficulty with finding out certain information.
Then of course the Relationship Finder is way cool and much more robust then the similar tool at the other websites. Allowing you to check the connection between any two profiles, not just your profile to others.
Lastly there is a cool tool I have found outside of WikiTree where you can upload your GEDCOM file and it makes a nifty family tree graphic for you. Check it out at http://learnforeverlearn.com/ancestors/