How does the Spanish future tense differ from the English future tense ?

What is the hardest part of speech for Spanish speakers to learn when learning English, and why?

First off, your question is subjective--what one Spanish speaker finds easy another may struggle with.

As far as grammar goes, English grammar is much easier than Spanish Grammar. Spanish verb conjugation and the extra verb tenses (subjunctive, imperfect) make Spanish a more complex language. Spanish speakers do struggle a bit with the difference between both make and do (they both mean hacer) and much and many (both meaning mucho).

Pronunciation and spelling, on the other hand, are a whole new ball game for Spanish Speakers. In Spanish, words sound exactly how they are spelled (imagine how much easier a spelling bee is in Spanish than in English). The whole concept of silent letters and multiple ways to spell the same word (i.e. bye, buy, by) is as foriegn to them as por vs para is to English speakers.

Some of the hardest things to pronounce are words: that end in '-ed' (past tense), 'th' (teeth pronounced without the -th sounds an awful lot like 'tits'), and words that begin with 's' (for some reason, Spanish speakers tend to want to add an 'e' before the s so it sounds more like eschool than school).