I've read Frankfurt's On Bullshit, and I'm halfway through Bullshit and philosophy by Hardcastle and Reisch (Editors). There's some good stuff there, but, like all philosophy, quite a lot of bullshit.
The first problem is that all the work I've read so far tries to define bullshit in terms of truth. Second, many of the pieces try to draw deep distinctions between bullshitting and lying. Third, many of the pieces try to make the descriptive sense of bullshit precisely match the normative sense. The first task is impossible, the second trivial, the third unnecessary.
Since nobody has any idea what truth is, it makes more sense to define bullshit in terms of falsity. On this view, bullshit can be relatively easily defined as any discourse that makes it harder to detect falsity. As such, a lie can be defined as a special case of bullshit: a statement the speaker knows to be false, and intends to convince the listener to believe as not false.
Statements can be bullshit on one level, and not bullshit on another level. Arbitrarily pick any layer of abstraction; if the statement makes it harder to detect falsity at that layer, it is bullshit at that layer. And bullshit is pejorative only to the extent that we are interested in detecting falsity.
When I say, "How are you?" and you say, "I'm fine," we are bullshitting each other: I don't particularly want to know how you actually are, at least not at the superficial level, and you don't want to tell me. But neither of us really cares about detecting falsity on that level, so the bullshit is benign.
On another level, the utterances "How are you?" and, "I'm fine," really are concerned with detecting falsity, and do the job well. I want to tell you, "I acknowledge your existence as a sapient human being and I'm interested, at some level, in your well-being." And you want to tell me, "I acknowledge your interest, and there is no immediate need for concern." If I didn't say, "How are you?" in situations where it was expected, you would detect that I didn't really care. We're not necessarily establishing "truth" — one or both of us might actually be lying — but we are excluding at least some possibilities, that either of us might be indifferent to the other.
Furthermore, at yet another level, the query, "How are you?" opens the door to a request for help, at least to the extent of requesting an expression of sympathy. While I don't necessarily want to hear every trivial complaint you might have, if something is seriously wrong, if you just found out you have cancer, or lost your job, or you're in jail and need to be bailed out, I'm seriously giving you the opportunity to ask me for help.
Likewise, fiction is bullshit on one level — writing a book of fiction doesn't help us detect whether or not the events depicted really happened — but that's unobjectionable bullshit: We don't care whether or not there really was a prince of Denmark named Hamlet. But fiction is not bullshit on another level: We really care that the events depicted could have in some sense happened. We get mad and feel cheated when an author gratuitously violates verisimilitude. That's bullshit on a different level than just mere fictionality, on the level where we really are interested in reading fiction to detect falsity. If an author is going to try to contravene our intuitive notions of verisimilitude, she has to convince us those intuitive notions are mistaken, and thus help, and not hinder, the detection of falsity.
We don't always have to avoid bullshit on every level. The search for truth (even by the indirect means of eliminating falsity) is important, but it's not all important. But contrawise, although the search is not all important, it's still important. And when we wish to be taken seriously on matters of truth and falsity, it's important to avoid bullshit.
Given this pretty straightforward understanding of bullshit and its normative implications, we can then catalog its various flavors.
As mentioned above, there's lying. You know it's false at some level, and you're trying to convince someone it's not false at that level. It's straightforward that lying hinders the effort to detect falsity, and a false statement is a lie (and thus pejorative) because by definition the listener cares about (or the speaker believes the listener cares about) detecting falsity.
Everyone understands, I think, the bullshit we see in advertising and public relations. As Frankfurt notes, these speakers really don't care what the truth is; they avoid direct lies, but they're trying to convince their listeners of propositions they don't really care might or might not be false. Indeed, they want to convince their listeners not to examine their claims carefully enough to detect falsity.
As mentioned in my previous essay there are ideas formulated so that it's impossible to detect falsity. Again, if you express yourself such that I cannot in principle detect falsity, you are hindering my effort to do so, and it's thus bullshit.
The kind of bullshit we typically see in a lot of secular philosophy is pomposity and obfuscation. If you make it difficult for me to figure out what you're actually talking about, you hinder my attempt to my attempt to determine if you're mistaken. It's bullshit. There's no excuse for this kind of bullshit. If physicists can make General Relativity and Quantum Mechanics comprehensible to the ordinarily intelligent reader with an interest in the subject and the willingness to put a little effort into comprehending the subject, there's no reason for philosophers to plead their subjects are too complicated for any kind of clarity. (Indeed, reading Bullshit and Philosophy, I kept imagining the scene from Monty Python and the Holy Grail where the crowd shouts, "Get on with it!" Too many philosophers seem to try to bore their readers into agreement.)
Simple mistakes are the weakest and least pejorative kind of bullshit. If you express yourself clearly, concisely, directly, and succinctly, you'll first of all detect falsity in your own thinking. And if you cannot express an idea clearly, you should be clued in that you don't understand it at all. If you do your best to be clear and you do make a mistake, you're at least making it easier for the reader to detect it.