I think:

- Pronounce of "R"

- use of "por" and "para"

- subjunctive

- use of "ser" y "estar"

- "Pretérito Imperfecto" and "Pretérito Perfecto Simple"

- Imperative

- verbs conjugation

@Dorothy: I hope not all people share your views about machismo and Spanish. I think your comment evinces some ignorance on your part and is perpetuating a stereotype.

The use of gender is not exclusive to Spanish; it's a feature of all Romance languages. According to your line of reasoning, any language who has this feature is also guilty of machismo. But this is obviously not true.

Furthermore, you seem to be lumping together all Spanish speakers when you say "the whole culture seems to be seeped in machismo". What "culture" are you talking about? There's not a common culture shared by all native Spanish speaker. This stereotype is very popular in the States, where they equate Spanish with Mexican or Mexico.

I don't know about other Spanish speakers, but I found your comment rather offensive.

Real answer: 

No problem with pronunciation. Spanish is so easy for that! 

Every word is written just the way it is said. Gotta love it! Easy. 

No problem with the subjunctive. I don't understand why so many Americans find that hard. If there's doubt, there's the subjunctive. I probably use it too much. ha. 

All the different kinds of Spanish accents are difficult. Probably no different than a Spanish speaker needing to understand someone from Brooklyn, someone from Ireland, someone from India and someone from England. Accents and dialects make it hard because you have to learn different idioms from all the different places and people from different places sound so different! 

Prepositions are always hard. Eeks. I get those confused all the time, but really, the WORST thing about Spanish is that you label EVERYTHING as female or male and it doesn't even make sense why things are labelled the way they are. The whole culture seems to be seeped in machismo. 

Machismo in general can make Spanish harder. Oh, the attitudes sometimes! Yikes. 

Get rid of all the sex-linked words and machismo and I think Spanish would be the easiest language ever for an American to learn. 

Chris there is a form of subjunctive in English but it doesn't involve a whole new set of difficult to remember conjugations. 

I would say learning all the tenses and their conjugations is one of the most difficult things about learning Spanish. It is more difficult than what is faced by Spanish speakers learning English. 

I am not a native English speaker but I am pretty sure that subjunctivo verb forms have a good chance of being a challenge for them, since those forms are used neither in English nor in Dutch, my native language. I had trouble with them.