key excerpt (but read the whole thing, it's very good if you like this kind of thing):
The Two Conditions of Intuitive
In our research, we've discovered that there are two conditions where users will tell you an interface seems 'intuitive' to them. It only takes meeting one of the two conditions to get the user to tell you the design is intuitive. When neither condition is met, the same user will likely complain that the interface feels 'unintuitive'.
Both the current knowledge point and the target knowledge point are identical. When the user walks up to the design, they know everything they need to operate it and complete their objective.
The current knowledge point and the target knowledge point are separate, but the user is completely unaware the design is helping them bridge the gap. The user is being trained, but in a way that seems natural."
It's a word I avoid myself, because people often use it to mean "better/easier" (e.g. Apple's software is "more intuitive" than Windows) and ignore the context, (e.g. Apple software is not more intuitive to someone using nothing but Windows for the last 10 years), but I find the framework given in my link is great for getting to the bottom of what's actually wrong when people complain about something being "unintuitive".