Direct competitor? I don't know. Upstream someone said Hero Labs.
The only thing I'm aware of that DDB does that Fantasy Grounds doesn't (and FG does a huge number of things DDB does not) is a comprehensive search (FG's is limited to the product you are browsing). And Manage Characters (FG just builds them automatically, it does not allow you to reduce levels or take away things without manually editing the character). I'm sure there are other things, but as I don't really find other things missing in FG, I wouldn't know. (Oh, FG doesn't have a wiki interface that Realms Works does).
Finally, you said the key words "the promise of". I used to subscribe to DDI back when the had "the promise of" a VTT, of homebrew, of... all the things that did not materialize.