As long as more people like the game than dislike it, then it is considered a success.
I am actually going to disagree with that. Many artists recognise that there is a large difference between something that is "Good" and something that is "popular". While there is certainly nothing to be ashamed about when you make something popular, or to be popular, it isn't nessisarily a success just because a lot of people bought it and mildly liked it.
Things can be a financial success but a critical failure and a lot of the time these differences start to appear over time as the people migrate and everyone left are people who would pay close attention.
in streamlining away it's problems (which even I admit they definitely have done with Skyrim), they also removed a lot of the things Morrowind got right. Now, even if this wasn't done to appeal to those "damn dirty casual players" it's still a problem. I would love to see a game with the engine of Skyrim, but with the depth of Morrowind or even greater depth. But I doubt it's possible.
Streamlining when done right is about doing away with details and mechanics that didn't need to be complicated. Streamlining done wrong (from a critical point of view) is when they remove depth and complexity.
Honestly I have no hatred for casual players but I really dislike how much the market is "Lowest common denominator" right now. Where everything needs to be distilled from any sort of challenge, complexity, or depth and made plain.
I know someone once challenged that old games didn't actually have depth but it was rather an exception... but I have plenty of games where it didn't pull any punches and you needed to know what you were doing and some were in fact "Triple A" games.
There really isn't anything wrong with Skyrim but it is a trend that will, to me, only get worse until even the casual players will get bored of it. Skyrim is fun but it shows how much the series has lost. The most impressive thing in the game are the invisability mechanics (which I still awe over) and quite a few of the missions would have been considered great missions no matter what game it was in.
Yet I feel like everything that was great and done well in Skyrim was something they are going to continue to streamline. For example Light and Heavy Armor being seperate skills? Well they are exactly the same as far as the game is concerned but can we simplify it further?
-I need to be careful what I say and not let my depressive side dictate this... because Skyrim is very impressive and my issues are definately not with the game in general. Yet I do honestly think that every problem introduced in Skyrim is only going to get worse. I do think the next game in the series, that isn't the MMO, will only get more bland.