When typing multi-line comments Visual studio used to add an automatic asterisk to each new line of the comment as I press enter(apparently 2015 does not do this anymore). I
This is a well-known problem, the feature fell by the way-side in the Roslyn refactoring of the C# Editor Features. Nothing to do with config or a change of preferences, it simply was not implemented.
It is currently being tracked by this Github issue. Status is "Up for grabs" right now with nobody having shown an interest in grabbing it yet as of today. Afaict, best to not hold your breath for it, I think they have several bigger fires to put out. It will surely be delivered in an update sooner or later.
Just wanted to update and say that this problem was apparently fixed(sort of) in one of the newer updates of Visual Studio. The editor still does not complete the comment for you, but it does at least keep the formatting consistent, and adds an asterisk to the next line for you.