Its an intense sensation, and it's possible to interpret it subjectively as just that and nothing more (speaking from personal experience). What more can one say about it? The more enmeshed one is in bare reality, the more painful it seems, logic will not help one realize that.
Well, at the risk of repeating myself (I have argued this many times, but then, I am on something of a mission to get people to see this),it goes like this:
this intense sensation at issue here, it is Bad, and I'll give it a capital 'B' to set it off from things like bad couches and wiper blades, things the badness of which is plainly dependent on something other than what they are, like a bad couch directs one to comfort, appearance and the rest. This is bad with a "B" is the ethical Bad (reading Wittgenstein's Tractatus, his Lecture on Ethics; reading Mackie's book, the one in this thread, is very helpful; also, getting familiar with phenomenology helps a great deal to remove philosophical thinking from the "naturalist" perspective of empirical science, as it recognizes the derivative nature of it). So the answer to your question "What more can one say about it?" is that the flame burning the flesh of a conscious person with an uncompromised nervous system is Bad. It's not about the act of doing this to someone and calling the act Bad; not yet. Before we get to the entangled affairs where defeasability steps in, that is, where one Bad mixes with the utility of others, Goods, Bads, whatever, we want to know what all the fuss is about in the first place. And the fuss's source is in the Badness or Goodness of things we care about, like not having your kidney speared.
The philosophical issue arises here, of course, when we ask the question, what is it? Two things present themselves: One is the description of the Bad event and the other is the event being Bad. The former is rather easy: the nerves are duly excited at the finger tip, exciting other nerves that excite more nerves in the brain, where the "event" of pain occurs. In this description, no matter how scientifically detailed, you will not find the ethical Bad. It does not present itself for objective observation. Empirically, it doesn't exist at all, and is not among the "facts" at all (Wittgenstein). Again: the event does reveal itself in extraordinary detail to the neurologist, but she will never locate this ethical Badness. That makes this, Mackie, says, queer beyond belief.
Mackie's conclusion is that there simply is no such thing as ethical Badness at all, just as there are really no Platonic forms or demonic possession. These are just fictions of bad thinking.
The reason I like Mackie is that he presents a strong argument, and he does it clearly, and when someone does this they are either going to convince you or they will strengthen your opposition, having looked clearly and integrated it into your thinking. For Mackie is simply wrong in his conclusion that there are no objective values. He is "absolutely" wrong. Observe the burning finger on your hand. This is a given in the world, not a confusion in thinking.
I will argue this further if you have an interest.