Although I love that you're a Tarantino fan, I will have to completely disagree with you on this.
It was a bit long, but from the perspective of Tonto's story, it was very effective. It was very nicely made and well-written for the most part. Some bits were too much (like certain jokes), but I thought this was a very good film from a filmmaking perspective. I think you were expecting something more along the lines of Django Unchained. When I went to see this, I knew it was going to be more along the lines of Dead Man, which it was to an extent.
Also, most of it would make more sense to an American audience given how grounded it is in the culture. The reason why most critics over here hated it however is because of it's subject matter. The film is very sympathetic towards Native Americans, whereas basically every Western that depicts them is against them. They are the bad guys- unintelligent and savage. But in this one, it's the White Man who is the antagonist (and for very realistic reasons given what happened during that time period in America, as well as the decades preceding it), and I guess that didn't sit well with these critics.
That, and the fact that The Lone Ranger (which was a big series in the 60s) was basically made sidekick (and given an origin story where he is less than courageous as opposed to his real "sidekick"). So old people didn't like that either. Personally, the title "The Lone Ranger" could apply to Tonto as equally as it applies to John Reid.