I think Casey Hudson is great and I respect his work on this game. But I think that EA should have thought through allowing Casey to create this "polarizing" ending. If I was an executive trying to create as much revenue from my product line as possible, I would be very wary of an ending that closed off the universe of my game from future installments. You can of course feel free to correct me if I am wrong, but it seems to me that the ending of ME3 does just that. Other franchises have made a vast majority of their money on the expanded universe of toys, games, etc.
On the flip side, if Casey was trying to create a series finally that was divisive and final, I think he was forgetting he was working for a huge cooperation, that will probably take his ending, and scrub it in favor of selling more games, and making more money. It is sad, but EA will probably release a DLC that will nullify what he was trying to achieve.
Slight *SPOILER* ahead
These are my thoughts from an unbiased standpoint. But as a fan, I feel cheated into believing that the adventure would never be through. I feel cheated, that I was led to believe that the Krogan would be a new threat, or the Racni may turn on the Galaxy once more. But with the destruction of the Mass Relays, it feels pointless to wonder or care. The doors to the universe slammed shut. I think every one would agree that the death of a hero is a hard thing to endure, but we understand that that all stories must end to let new ones begin. It's the death of a universe which means the death of all stories and this has polarized us. Mass Effect has never been the best thing I have played, but it had something that make for a good adventure story; A setting that could revival Star Wars or Star Trek in scope and depth. You can kill Shepherd but why did you kill Mass Effect?