Anthony Bennett and their supporters spring to mind immediately upon reading Donna's guide.
Arrogance. They are always fact-seekers, questioners, people who are trying to discover the truth: sceptics are always "sheep", patsies for Messrs Bush and Blair etc.
Relentlessness. They will always go on and on about a conspiracy no matter how little evidence they have to go on or how much of what they have is simply discredited. (Moreover, as per 1. above, even if you listen to them ninety-eight times, the ninety-ninth time, when you say "no thanks", you'll be called a "sheep" again.) Additionally, they have no capacity for precis whatsoever. They go on and on at enormous length.
Inability to answer questions. For people who loudly advertise their determination to the principle of questioning everything, they're pretty poor at answering direct questions from sceptics about the claims that they make.
Fondness for certain stock phrases. These include Cicero's "cui bono?" (of which it can be said that Cicero understood the importance of having evidence to back it up) and Conan Doyle's "once we have eliminated the impossible, whatever remains, however unlikely, must be the truth". What these phrases have in common is that they are attempts to absolve themselves from any responsibility to produce positive, hard evidence themselves: you simply "eliminate the impossible" (i.e. say the official account can't stand scrutiny) which means that the wild allegation of your choice, based on "cui bono?" (which is always the government) is therefore the truth.
Inability to employ or understand Occam's Razor. Aided by the principle in 4. above, conspiracy theorists never notice that the small inconsistencies in the accounts which they reject are dwarfed by the enormous, gaping holes in logic, likelihood and evidence in any alternative account.
Inability to tell good evidence from bad. Conspiracy theorists have no place for peer-review, for scientific knowledge, for the respectability of sources. The fact that a claim has been made by anybody, anywhere, is enough for them to reproduce it and demand that the questions it raises be answered, as if intellectual enquiry were a matter of responding to every rumour. While they do this, of course, they will claim to have "open minds" and abuse the sceptics for apparently lacking same.
Inability to withdraw. It's a rare day indeed when a conspiracy theorist admits that a claim they have made has turned out to be without foundation, whether it be the overall claim itself or any of the evidence produced to support it. Moreover they have a liking (see 3. above) for the technique of avoiding discussion of their claims by "swamping" - piling on a whole lot more material rather than respond to the objections sceptics make to the previous lot.
Leaping to conclusions. Conspiracy theorists are very keen indeed to declare the "official" account totally discredited without having remotely enough cause so to do. Of course this enables them to wheel on the Conan Doyle quote as in 4. above. Small inconsistencies in the account of an event, small unanswered questions, small problems in timing of differences in procedure from previous events of the same kind are all more than adequate to declare the "official" account clearly and definitively discredited. It goes without saying that it is not necessary to prove that these inconsistencies are either relevant, or that they even definitely exist.
Using previous conspiracies as evidence to support their claims. This argument invokes scandals like the Birmingham Six, the Bologna station bombings, the Zinoviev letter and so on in order to try and demonstrate that their conspiracy theory should be accorded some weight (because it's “happened before”.) They do not pause to reflect that the conspiracies they are touting are almost always far more unlikely and complicated than the real-life conspiracies with which they make comparison, or that the fact that something might potentially happen does not, in and of itself, make it anything other than extremely unlikely.
It's always a conspiracy. And it is, isn't it? No sooner has the body been discovered, the bomb gone off, than the same people are producing the same old stuff, demanding that there are questions which need to be answered, at the same unbearable length. Because the most important thing about these people is that they are people entirely lacking in discrimination. They cannot tell a good theory from a bad one, they cannot tell good evidence from bad evidence and they cannot tell a good source from a bad one. And for that reason, they always come up with the same answer when they ask the same question.