My learning background is decidedly not Rav Kook-oriented; his mashiach-oriented view of Israel/world/history, and his chassidic/mystical view the individual neshamah and the national neshamah don't mix well with the sources which have framed my way of looking at the world.
Nonetheless, I find Rav Kook's writing compelling, on many levels.
One of the elements I truly appreciate is the way Rav Kook is unapologetically fierce in his views, judgmental to the hilt, and yet, simultaneously, he is honest about the good he perceives in those whose views differ from his own.
This is especially clear when Rav Kook describes the secular chalutzim (pioneers) of his day. He tells you exactly what he thinks about their religious views, even as he expresses great respect for their traits which he considers admirable.
Case in point - Chazon haGeulah pg. 102-103, 105:
Among many of our generation, we find that despite their descent from recognizing the content of the soul of the Torah, and despite the fact that they have strayed from the path of Gd and rejected religious law and sanctity and selected the path of the heretics of the nations, with all of that, social justice lives in their hearts in a great and mighty way, and they adhere to the rational and ethical mitzvot to the point of surrendering their lives…
Let us now gauge their level of ethics relative to the humaneness of the masses of every nation and tongue, who dwell upon their own land, and relative to the state of our own nation in an earlier generation.
We must acknowledge that they stand on a level which is so elevated that it would be fitting for us to praise them before the eyes of the entire world.
This stands in stark contrast to the popular approach to embracing those with whom we disagree. Many rabbis today seem to feel a need to eliminate their own judgment in the name of pluralism, to avoid all criticism in order to work in partnership with those whose views differ.
Not so Rav Kook. Rav Kook is often portrayed as a grandfatherly, Wilford Brimley-esque (or perhaps Carlebachian) rabbi standing arm-in-arm with shirtless kibbutzniks and smiling upon their secularism, but that's not the picture that emerges from his writing. He calls it as he sees it. "I think they have descended and have embraced the paths of heretics - but I love and admire them for their social ideals."
This is openness I can believe in, a warm and sincere appreciation expressed with authentic sincerity.