I think Destiny is the equivalent of Mass Effect 1. A flawed new IP but with lots of love and care put into it, it just needs a sequel to really work the kinks out.
I get your point, but Mass Effect isn't the best comparison. Many fans of the series think that was the best game and it was much better received than Destiny. I think Assassin's Creed would be a better comparison.
I'm not going to pass full judgment on the game yet because I'm only about quarter way through Venus, but overall I'd rate my experience as a 6-6.5. When I'm playing with friends, it's like a 7.
I think the problem with Destiny is that many feel like they were misled on how the full game would actually be. Bungie claimed that the beta was less than 10% of the full game and that there would more to Old Russia, which was a flat out lie. Destiny seemed to be so much more ambitious than the actual product we got especially when considering Bungie's pedigree.
Another thing is we just got Diablo III:RoS less than a month ago, which was far superior(IMO of course). Perhaps, if it wasn't for the marketing, Bungie name, and recent comparable games, many wouldn't be as down on the game as they are now.
It was hyped as if it was a new Halo and called the first true current gen game even though it's on last gen consoles. Things like this raised people's expectations higher than they should of been based on what was played in the Alpha/Beta.
Most of the problems we're talking about now were also present in the beta, but we chose to look past them, hoping the full game would be different based on the above factors(myself included).
Sure, Destiny is a competent game and it's fun when played with friends, but really you can say that for almost any non-shovel ware game. Destiny's content is NOT what many were expecting and the reactions are basically proof of that because many played the Alpha and/or Beta and are still disappointed in the full game.
Perhaps, you can say we had delusions of grandeur expecting Destiny to be a wonderful cross of Mass Effect/Borderlands with gameplay as strong as Halo's, but that's why there is so much backlash. It's not because it's necessarily a bad game, but it turned out to be merely a mediocre/average game that was a far cry from expectations.