Thoughts after reading Scholem's "Major Trends in Jewish Mysticism".
The basic chakira (set of possible conceptual frameworks) for (Jewish) mysticism is as follows:
The point of mysticism is to experience God's presence. There are two philosophies which allow this presence to be approached.
1) You can believe that God is already present in the world
2) You can believe God is about to enter the world, through the coming of the Messiah
The problem with #1 is that it is almost inherently pantheistic, and very quickly brings you close to heresy. Perhaps the Zohar itself has already crossed the line in this regard.
The problem with #2 is that messianic movements very easily get out of control in both the social and religious senses. Thus the Ari's messianic philosophy became the basis for that of Shabbetai Tzvi.
The restrictions on learning kabbalah (be over age 40, be married, etc.) are attempts to avoid these inherent problems. Often, it seems, these restrictions haven't been enough.