For a native English speaker to learn Spanish or for a native Spanish speaker to learn English. Do you think about?
I don't think Spanish pronunciation is difficult. With the exception of 'r' and 'rr' we have the same sounds in English, and words are said as they are spelt. Speaking naturally and quickly is hard because the rhythm and cadence of the language is very different to English, but pronunciation itself is not that hard. You can encounter a word for the first time in Spanish, and know how to pronounce it because of this. You often can't do that in English. There are what, five vowel sounds in Spanish? In English we have the five vowels but 12 different ways of pronouncing them.
Well, here's how I'd break it down
everything related to spelling
finding material to read, listen to, watch... On the internet
regional differences in the way it's used
Equally difficult going both ways
use of prepositions
phrases involving time ("desde hace cuánto tiempo" vs "how long has" for example)
The English-speakers who find it hard to learn Spanish are those who have no knowledge of any other language, and may even have limited knowledge of the workings of English grammar. A monoglot British or American person may find Spanish a challenge because they have fixed ideas and narrow-minded language habits with regard to word order, for example, and they may struggle with basic 'alien' concepts such as gender agreement of adjectives and past participles.
However, for an English-speaking person who has learnt other foreign languages and has an understanding of grammar and structure, Spanish is actually relatively easy. The grammar is logical, and the spelling/pronunciation relationship is wonderfully straightforward.
When it comes to vocabulary, the English-speaker learning Spanish has an advantage over the Spanish-speaker learning English. English has a vast number of words of Latin origin, and even though these aren't always used in everyday contexts, any educated English-speaker will know and understand them. This helps us a lot with Spanish vocabulary. By contrast, a Spanish-speaker has no clues at all in their own language to help them with the thousands of Anglo-Saxon words which English uses in everyday contexts.
Personally, I have never had a single Spanish lesson in my whole life, but Spanish does not seem the slightest bit foreign to me. I can manage on a basic level when I go to a Spanish-speaking country, and I can read and understand most things written in Spanish.
The only real advantages that the Spanish-speaking learner of English has over the English-speaking learner of Spanish are those factors which are external to the language itself : incentive and exposure. Because of American-led globalisation, there is motivation worldwide to learn English, and an enormous amount of English 'out there' internationally.
@Mike - just to be clear; are you saying, for example, English pronunciation is harder for a native Spanish speaker that Spanish pronunciation is for a native English speaker? If so, I find it hard to believe. If, on the other hand, you are saying that English pronunciation is "globally" harder than Spanish pronunciation, then that is clearly false.
@Geovanna - in general, it's easier for a Spanish speaker to learn English, due to more exposure and stronger sources of motivation.
To put it as simply as possible, English has more sounds that don't exist in Spanish than vice versa. The rolled R is the main thing that English speakers have trouble with (I personally don't think it's very difficult) and a lot of English speakers don't understand the need to change intonation, word stressing, tone... So they sound really stiff and unnatural speaking, but when spoken at a slow enough speed (and clearly obviously) sounds are easier to distinguish for an English speaker. Even just going from Spanish to Portuguese it's obvious to me, at least, how much easier Spanish pronunciation is than Portuguese. Certain Argentinian and Spanish accents, for example, can be a little trickier, but viewed as a whole I really don't think that the pronunciation of Spanish is that difficult. With the possible exception of Italian, I'm not sure that there's an easier language for a native English speaker with respect to pronunciation.
In a similar manner, from my very limited knowledge on the subject I would argue that Japanese pronunciation would be much easier than Chinese for an English speaker. Going the opposite direction I think that the difficulty is similar.
Aspects of languages aren't always equally difficult going both ways.