History of Philosophy, as a discipline, involves a lot of technicalities -- every kind of technicality, in fact, because they all show up somewhere in the actual history of philosophy or its study. But there's also a sort of je ne sais quoi quality to good HoP-work. Obviously, since it's je ne sais quoi, I don't know exactly or completely what it is, and don't think anyone else does. But bits and pieces of it can be brought out. I was thinking of one way to do it the other day; one hard-to-pin-down aspect of HoP is grasping the spirit or sense of a time, a movement, or a philosopher. On the one hand, it's an absolutely essential skill, which can save you from many of the worst kinds of errors. On the other hand, there is no method to it, and no definite way to learn it. This makes it one of the hard things about study the history of philosophy.
An example might help. I once ran across a website, by a professional philosopher, for one of his Intro or Ethics classes (I forget which). And he claimed that Kant held that the Golden Rule was a "deeply misguided principle." Now, this and this alone would immediately set alarm bells ringing for anyone who had any sort of sense of Kant. It's just not the sort of thing Kant would ever say or commit to. The Golden Rule, of course, was stated by Jesus, and to put it very baldly, Kant would never contradict Jesus. If Kant did have a problem with the Golden Rule, anyone with familiarity with Kant knows what he'd do: he'd argue that it was an excellent principle but only for a very specific domain, or he would say it had been widely misinterpreted and give his own interpretation. It is in fact difficult to get a sense of what Kant's view of the Golden Rule is, because he has only scattered comments that can be considered even relevant -- but having a sense of Kant, grasping the spirit of Kant, would have prevented this particular philosopher from an egregious misinterpretation.
Or consider the Scottish Enlightenment. One of the difficult things about interpreting the Scottish Enlightenment are the overwhelmingly important roles played by law and aesthetics. Jurisprudence and good taste -- each of them affects almost everything in almost every major philosopher of the Scottish Enlightenment. This is especially difficult for us because in general we don't live in an intellectual world where law and taste are the governing principles of most of life. But it can prevent you from making a serious mistake, like the old view that Thomas Reid just 'tacked on' the Essay on Taste to his book on Intellectual Powers, or like not noticing how much of Hume's account of morality is actually an account of law. Or, to take two other examples from the same period, it's difficult to grasp the complicated balance between French intellectual life and English intellectual life that governs so much of Scottish intellectual life, or the utter urgency with which almost everyone insists that there needs to be some kind of middle-of-the-way between enthusiasm and priestcraft.
There's no easy way to learn these things, or even determine what will be important for diving in. It makes HoP a perpetual adventure. But it trips everyone up sometimes, and everyone comes to a point where they realize that some previous view they held was not even remotely plausible, however much it looked at the time like the text supported it.