English, like all languages, has trade-offs. It's a very 'colourful' language with many similar synonyms and a degree for many concepts (boiling, hot, warm, luke-warm, tepid, cool, cold, freezing...) but this means there is a lot of vocabulary to memorise (I have heard that there are more words in an English dictionary than any other language).
Grammar is difficult for learners of non-latin-based languages - in Thai, changing the tense is uncommon and only then requires the addition of a prefix, suffix or tense-determiner which is very similar for Chinese and Japanese, in English there are many tenses (perfect / simple / continuous for past, present and future...) so tenses can be tricky to get right for the non-native speaker. At least there are almost no gender-specific grammar rules or, as ichbinlance points out, so many conjugations.
Spelling is another issue. American spelling is objectively easier than British spelling but is still not entirely phonetic (there is a seperate phonetic alphabet found in dictionaries that is worth learning if you want to teach English to Asian students - in my experience they prefer it as would I). Although the spelling is based on an alphabet which is easier than Chinese - memorising 26 characters and their sounds is easier than memorising 2,600.
All in all (a phrase that itself would need an explaination!) I have found English to be challenging to teach and I often imagine that if I were to learn English as a non-native speaker then I would struggle with the above issues.