I could mention Everything, I think the only easy thing in Russian is the nominative case :DDDD But ok, I haven't touched all the aspects of the grammar but for sure, " accusative plural animated " and " genitive plural " and the "numbers" are a nightmare for me, they are the reason I always think of giving up on learning your language ;)

Правила образования формы деепричастия от некоторых глаголов... Или тупо их правильная форма. =)