Me, as a Brazilian with lots of experience with Spanish, the hardest part is to make my speech as much redundant as possible like Spanish speakers love to do! ;D

From my experience with people learning Spanish, definitely differentiating between "ser and estar", "pedir and preguntar". In terms of grammar, the hardest part seems to be the subjunctive and the irregularities of the preterit (also choosing between that and the imperfect). 

I think the hardest part about it is that it`s a language which one has to learn how to "feel" it. The subjunctive ser/estar are good examples for that but there are truely a lot more of them. It`s actually the first language which I decided to practice by talking to native speakers. I still think that grammar is important, like you can see in many other comments I wrote, but this language requires other methods (at least it seems so to me) because it has this special "character". I am still trying to figure out what`s the best way for me, however, talking to native speakers in order to get used to the "feeling" was a good option. I don`t regret it so far!

Sujunctive, imperfect, ser & estar, por & para... Vocabulary is an issues as there's so much to learn but understanding is not the issue, it's simply memory.

 spanish grammar is  the most difficult for me