I just got back from dinner, but apparently people have begun discussing it in comments at this relatively unrelated thread.
I don’t know much about it, so all I’ll say is that if they had a first-stage engine problem, it proves out their design capability to have mission success with engine out.
Clark Lindsey has some notes from the post-launch press conference.
[Update a few minutes later]
I would go further, and state explicitly that this increases my confidence in their vehicle. It was an inadvertent flight test of a capability that they would hope they don’t have to use, but now know that it works.
[Update a few minutes later]
Never mind my first response to Paul Breed in comments. Apparently it wasn’t just an engine shutdown — a first-stage engine exploded.
That puts a different complexion on things.
Now the question is, what is different about that Merlin than the one on the second stage, other than nozzle size? Well, there’s the environment. It occurred during Max Q, so the second stage wouldn’t have to worry about that.
If it had happened on the second stage, would it have just resulted in a lost of thrust (that is, does the second stage have a similar shrapnel protector) or would the stage itself have exploded?
If the former, it would probably be survivable (that is, payload or crew recovered) without an abort system, though a mission failure, so a concern for payload customers. If the latter, it might have happened too quickly for an abort system to be activated. It all depends on how much warning they had that the engine was going south. But obviously this would be of concern for commercial crew. Of course, as discussed in comments, this engine version will be retired after the next flight, currently scheduled for January, so if that one goes all right (and they certainly won’t fly until they understand what happened to this one), it will have retired with a 98% demonstrated reliability, and the Merlin 1D will have a clean slate on the following flight.
[Monday morning update]
Charles Lurio notes in email that one difference besides nozzle size (and environment) between upper and lower stage engines is the lack of active cooling in the nozzle of the latter. This would only be significant if the failure was caused by a problem in one of the cooling channels.