I don't think I fully understand what Nagel means by developing objective tools to describe experiences, I should re-read the full article a few more times. I think, though, that you're exactly right: We _do_ have a shared understanding and experience of what its like to see a red ball, and we can talk about it in the abstract. My reading of that is that because we do have a shared human experience there can and indeed must be a physical account (Nagel's term) of it. Nagel's argument brings in a wedge of doubt about whether that shared experience itself is real by saying, at least partially, that because we can't explain what it is like to be a bat (bat point of view) in human terms we won't be able explain what consciousness is for humans _or_ bats with a physical account (at all or at least right now). Dennett, I think, says that that is more or less backward, and that all the means is there are different kinds of consciousness. In the article I mention he says,
"The big mistake we’re making,” [Dennett] said, “is taking our congenial, shared understanding of what it’s like to be us, which we learn from novels and plays and talking to each other, and then applying it back down the animal kingdom. Wittgenstein”—he deepened his voice—“famously wrote, ‘If a lion could talk, we couldn’t understand him.’ But no! If a lion could talk, we’d understand him just fine. He just wouldn’t help us understand anything about lions.”
“Because he wouldn’t be a lion,” another researcher said.
“Right,” Dennett replied. “He would be so different from regular lions that he wouldn’t tell us what it’s like to be a lion. I think we should just get used to the fact that the human concepts we apply so comfortably in our everyday lives apply only sort of to animals.” He concluded, “The notorious zombie problem is just a philosopher’s fantasy. It’s not anything that we have to take seriously.”
I found that convincing, I'm curious if you do as well?