There's no single set of criteria for calligraphy - in particular, clearly the hiragana of Heian times is working on a very different basis from the Zen calligraphy of later centuries. Nonetheless, there is some commonality. Such considerations as balance, linework and composition always apply, in some way or other. Calligraphy should be done without hesitation, with speed and confidence, but never easily - one book claims "It is said of one master that he would never accept the work of any pupil who did not get drenched with sweat through the spiritual exertion of writing."