One part is rejection of modernism and romanticism of a fantastic past similar to fascist movements. Other is anti-colonialism, but only in the context of european colonialism, not muslim colonialism, which is fine. Because of the aforementioned romanticism to the middle ages, part of any Islamist project is creating a Caliphate, and it is easy to see in Islamic history that these were boundless.
The reason Palestine may be important for them is that according to their perception, while european colonialism is a humiliation, there is no greater shame than the existence of Israel, as it is no some vast British Empire, but rather a nation built by refugees and therefore weak by definition, thereby enhancing their defeat, which in their mind has religious implications. As Islam is a conquering religion, and their conquests are a proof of Allah's power.
