DDD modeling for a User Voice-like system
The design looks solid, a few thoughts though:


I would split IUserVoiceStore into more granular UserRepository and SuggestionRepository. Also, UpdateSuggestionAsync() seems to indicate you can only update a suggestion and nothing else at a time, which can be limiting. It also IMO goes out of a repository's jurisdiction to flush a specific object to persistent storage. Using some kind of separate unit of work class where you can put multiple objects to be updated as part of a business transaction might be a better idea.
Not always feasible, but maybe change the type of link between Suggestion and User from a full reference to just an ID to avoid the temptation of manipulating 2 aggregate roots at the same time. (I don't agree with the article you linked to in that regard)
Keep an eye on suggestions with a large number of comments - depending on concurrent access, they can clog up your system and cause locking problems, especially if comments become more sophisticated, with images and so on.