I guess I might as well add my two cents about the end.
Shepard having to sacrifice herself doesn't bother me at all. All throughout the game, I had allies and friends sacrifice themselves to save the day, and on one occasion, it was the direct result of encouragement from Shepard. Between that and all the goodbyes to allies and squadmates before the very end, I saw it coming.
I will say that I completely didn't buy Shepard just accepting what the Catalyst said. Shepard has got to be one of the most argumentative protagonists ever created. She gets into arguments with people over their methods, butts into the squabbles of complete strangers, tries to question and debate reapers, and actually had this exact same conversation with Javik earlier. I don't care how grievously wounded she was, it's completely unbelievable to me that she would stand by and let the Catalyst say things that been proven (through peace with the geth) to be patently untrue. Or if I hadn't been able to achieve peace, I should have had the option to agree or disagree all the same.
About the destruction of the mass relays, I have mixed opinions. I've heard about all the different ways galactic relations can go to shit leading up the end. In a story where things went really badly and all the species were at each others' throats, I could see destroying the relays and stranding everyone where they were as possibly more desirable than the all-out war that could be threatened otherwise. Doesn't bode very well for everyone that would be stranded on Earth, but whatever. Problem is, that isn't the story I played. The story I played involved healing centuries to millenia-old rifts and rallying every advanced race together to take back the galaxy and usher in a golden age. I actually feel like the question of whether the mass relays should be destroyed would be a great final choice and would be fitting for Shepard's last act.
I don't really have much to say about the choices the game does give you, since the actual endings are so similar. The Synthesis ending feels like it would be a great jumping off point for a completely different story. The only other thing I can think of is the complete lack of closure for everyone else. During the final battle, there should have been a scene for each species I had allied with where they do something incredibly badass that helps turn the tide, and after the end, there should have been a scene for every major character and species where I can see what happened to them and where they plan to go from here. They put so many alternate story outcomes and acknowledgements of what you had done in the other games, and seriously, it's the fucking finale. Not having any of this just reeks of laziness to me.
The ending didn't ruin the game or the series for me, I just found it disappointing. If they really are doing an add-on that adds more endings or adds more closure or whatever, I'd be interested in seeing it.