The episode was good..kinda bold to kill off 3 characters in one episode..something about season six of shows and having characters killed off. (the episode "The Candidate" from Lost season 6, "Seeing Red" from Buffy come to mind) but 'And Then There Were None' did seem a little weird throughout..like sam and dean weren't taking it seriously or something..idk. there was something off though.
When the Mother of All called herself Eve I didn't really know what to think. I expected thats who it was..but the explanation wasn't really there so I guess that's whats frustrating the most out of the episode. I thought about it and kinda figured maybe it has something to do with Eve eating the fruit and technically creating sin...but i dont know how that turns her into the mother of all monsters. Maybe it's from Adam and Eve's kids having kids and so on..eventually created a monster lol.
overall i'd give the episode an 8.5-9.0 IMO..the best episode of the season so far is 'Appointment in Samarra'. It just sticks out the most to me so far. Hopefully the two hour finale will do this season some justice