- no gender (le and la in French; der, die and das in German, etc.)
- no plural
- no articles (a, an and the in English)
- only three irregular verbs (corresponding to 'be', 'do' and 'come')
- only five vowels
- no diphthongs
- no consonant clusters (e.g., thrills in English)
So why is it considered a "difficult language"?
It is certainly different. For example, concepts such as politeness and probability are more important than in English. Often the subject is not expressed, so you have to make an intelligent guess.
And then there is the writing system. It is based on Chinese and the basic set now consists of
Grammatical patterns and examples