Even more than its vocabulary, it is Esperanto's grammar and rules which makes it exceptionally easy. Unnecessary complications have been eliminated: there is no grammatical gender, the word order is relatively free, etc. The rules have also been simplified as much as possible: there is only one verb conjugation, all plurals are formed the same way, a prefix can be added to any word to change it to its opposite (good/bad, rich/poor, right/wrong), and so on. Thus, after perhaps 30 minutes' study, one can conjugate any verb in any tense. This is a tremendous simplification compared to national languages.
Esperanto's flexible word-order allows speakers from different language families to use the structures with which they are most familiar and still speak perfectly intelligible and grammatically correct Esperanto. This also makes Esperanto an excellent translator of such different languages as Chinese, Japanese, Latin, English and French.