6/10??? Of course that everyone has their own opinions, but I rank it easily between 8.5 and 9. The aesthetics are gorgeous, the audio is always well suited to each and every situation (the narrator does a superb job), the gameplay is responsive and actions do feel like things are being done and having them done to the character. Getting all the upgrades can be quite challenging as well as the arenas, like MorbidToaster said.
For instance, I find that a game like Limbo is also nice but won't be anything beyond 7, as I find it too much of an interactive psyche-art app with some gameplay and puzzles put alongside. I did enjoy it though, and while it's an apples to oranges comparison, I enjoyed Bastion far more.
In terms of difficult games, I was playing The Binding of Isaac for quite some time, it was oddly compelling, and I could give it a 7.5.