As much as I would have loved a much more climatic final boss fight, I was not disappointed by the ending, because to me, the ending starts when you start your race to the temple of Mythal. You have your army fighting. You have your allies joining the fight, you get a great cutscene just before entering the temple (seriously seeing corypheus get nuked by those statues was really fun). The whole quest in the temple was really good (seeing the ancient elves - or their slaves actually), if you take Solas his comments throughout the mission are really enlightning. Then there's the Cory's General fight and the well.
AND THEN... you go back to Skyhold.... Thats the problem for me. Cause makes you think the ending is still 1 full mission away when actually you are just 1 fight away from the end. Granted that last fight could have been much more climatic and epic, and it falls short, but then again after beating Cory's back at every turn I wasnt expecting him to put much of a fight, I was even hoping to have a dialogue option where i could mock him silly into a nerdrage.
I'm not saying I wouldnt want it to happen, but i cant see how people were expecting a siege on Skyhold after the temple of Mythal. Cory lost that fight, you killed his general and most of his red templars, he had no manpower to directly assault Skyhold.
If the ending mission started with the temple and ended with Cory's fight, I think many people would say the ending was not as bad as they are saying.
But this is just my opinion, and ofc I'd love an ending with as much impact as ME2 ending, but well.... This is what we got, and while not AMAZING, i certainly wasnt expecting much more of the ending than what it was. Maybe DLC will bring something extra to Inquisition on this department, and I'm sure the villain of the next game will be more epic and badarse (paraphrasing Varric - Duuuummmmaaaat! DUMMMMAAAAAAAAAT, or is it Dread WOLF, Dreeeaad WOOOOOLLF)