I like that it is free for basic use, as well as being able to have multiple users in one account without too much hassle or cost involved with their service.
The fact you can't really customize what information gets displayed when someone comes across something new within a document (aside from having more than 1 page). If they would allow us to do this then we could make our documents even better! This also makes them very slow at times if there are many different people editing each other's work - which isn't always ideal either but sometimes it happens anyway so not sure how helpful/useful this actually proves to be unless used properly?