1. Easy to use. Once you get past the learning curve, it is much easier to use than any other tree (and I've used/use Ancestry, MyHeritage, Family Tree Maker, and FamilySearch.org trees).
2. One profile for each person. No duplication of efforts. No 50 trees with the same person. Just one profile for each person.
3. Sources. Wikitree requires you to use sources. To add a fact, you have to be able to back it up.
4. Collaboration. We're all working together to make Wikitree useful.
5. Connection. During the last month, because of profiles I created, I've been able to connect cousins with each other.
6. Projects. The cemeterist project is my personal favorite, but projects give me a chance to collaborate with others on areas I'm interested in, or have expertise in.
7. Responsive support. In the forum, G2G, anyone can seek assistance; in fact, there's a system of Wikitree-ers who help each other.
8. Responsive technology people. Someone mentions, "Gees, I wish we could do this," and next thing you know, we can. New innovations such as Wikitree X, a Chrome app, make Wikitree amazing and indispensable.