A few thoughts...
There are people that watched the show because they were intrigued by the Island and all the mysteries. This show made the Island and the mysteries a huge part of the show. The characters took a back seat these viewers. When you get no answers or answers like "they are just numbers," of course you are going to have people who are upset. I've heard people say they might as well watched Grey's Anatomy or something if they wanted to watch a show so focused on characters and their problems. What is the point of all the mysterious plot devices if you aren't going to answer them?
Luckily for me, I got into Lost because of the characters, even though there were pretty stereotypical. I was intrigued with each of their lives and wanted to know if they would ever get off the Island. The many mysteries were an added bonus for me. I am disappointed some stuff I wanted to know about never was answered (Walt!/"claimed" or "sickness"), but it wasn't that serious.
I have mixed feelings on the ending though. It was very creative and an unexpected twist that nobody saw coming. It did bring closure and explained a lot. To be honest though, I just didn't like the idea personally. I felt this "ALT" world was unnecessary. They could have came up with a better ending. The idea of being in a "limbo" they created, then having to have contact w/ other Losties just to move on to the afterlife is just a corny concept to me. The "ALT" world also made me so dis-attached from the characters on the Island during the Season. I didn't have any empathy towards the characters like previous seasons because there was this "other world." Sun and Jin died...felt nothing. Sayid died...felt nothing. They made the scenes really emotional, but I didn't know if they were really dead or alive because of this "ALT" world.
As far as Season 6, blah. All of the on-Island stuff was dope. Just wasn't feeling this "ALT" concept. It was just irrelevant and unnecessary. It just reeks of the writers thinking of a creative way for a happy ending. Pretty much how I felt. Enjoyed the ride. Nothing will come close to the epicness of Seasons 1 & 2. Too bad the ending was disappointing.