Learners - what's the hardest part
It's 40 years since I learnt Spanish on the job so I have a hard time remembering how it was but...
If you're a typical SpanishDict learner what gives you the most problems?
Some candidates might be:
reflexive verbs
word order
using pronouns correctly (round about four separate sub-topics here)
use of tenses - not just conjugations but when to use, for example, a historic present
other things that I'm too idle to think about just now
26 Answers
I'm also not your "typical SpanishDict learner", but I am spending a lot of time with new Spanish learners. It seems that the first difficulty they have is ser and estar, and when each is used.
They soon get past that, but then they have trouble with all the conjugations, especially when to use preterite or imperfect. My wife often complains about the "little" words: por, para, que, tal, etc.
I think the last big hurdle someone has (at least English speakers learning Spanish) is the subjunctive. Once they've jumped that barrier, it's just a matter of practice, practice, practice. Having an understanding and getting everything into the head is one thing. Actually using the language and getting all that knowledge out of the analytical part of the brain and stored in the language center is the final step (and should be part of the first step). You can't learn to ride a bike by studying; you have to get your hands on the grips, you feet on the pedals and the road rash on your skin before you can ride.
Pleasing Heidita 
I can study and learn verb conjugations, the differences between por/para or ser/estar, but the thing that gets me is all the tiny little words that I'm never sure whether they belong in the sentence or not. Pronouns and prepositions, mostly. Por ejemplo, "Hay una cosa de la que me parece importante tener conciencia..." Por qué no dice "Hay una cosa que me parece..."? Por qué se necesita "de la" en este frase? No tengo la menor idea.
Also, the accents. Like is it "¡Qué rico!" or "¡Que rico!" I'm not asking a question, but I still think it should be qué but maybe I am wrong. 
These are all just things that require practice, I think, and acquiring a feel for the rhythm and flow of the language.
Pronouns - when they are present they are a nightmare (se, le, lo, la - combinations are even worse)
Pronouns - when they are NOT present (the "Usted" form is a nightmare for me - who are you talking about? Me? Him? The dog? Arrrrgh)
The rest I can just about cope with. Let's be honest - there are a lot of things about Spanish that are a Godsend - phonetics, orthography, the uncanny logic to everything etc.
Sorry for the rant. Good question, though. 
I don't think I fit the bill as a "typical SpanishDict learner"; but the thing that pops to my mind is one that's not on your list, Geof: "getting over the shyness and just going for it".
I think most people get stuck, sooner or later, because they are too afraid to let go of their inhibitions and just get out and speak, make mistakes and learn from them!
There's an easy part!!???? 
La lengua.
Personally, I find the tenses to be one of my struggles. I seem to have a hard time keeping some of them straight and keep mixing them up.
Formerly, my biggest issue was just sticking with the learning process. I have been studying on and off for years, but not with consistency. This site has helped me take huge bounds in the right direction. Thanks, SpanishD!
Indirect objects would also be on my list. I know how to find the indirect object (if I am studying a sentence), but adding in the pronouns will take some work on my part.
The most difficult part in learning languages is in my opinion the vocabulary. It is the main problem which keeps me from understanding texts.
Building up a basic vocabulary is very tedious and time consuming, but once you are able to understand the words in a text without looking into a dictionary for every second word, it will also help you in learning the subtleties and rules that you didn't fully understand yet.
I think, most people give up on learning languages when they realize - after they start the first lessons and making rapid progress at first with grammar and simple sentences - how far they are from being able to understand the language because of lack of vocabulary.
In philosophy there is a very popular position that you are what you native language is. As Spanish is not my native, sometimes I just get lost in contexts.I always ask myself why it's like this, and not like that. On the other hand, it is very interesting to search for semiotics and origins of the words.
The biggest problem for me is that I won't be able to express myself as I can in native language. With all hidden meanings and shadings.
I think we are a bit not we, when we speak in other languages. 
My biggest problem is definitely actual speaking. I think gaining an ear for a new accent is one of the biggest challenges in any language, let alone a tongue for one. I always find it much easier to have IM conversations in Spanish than it is to actually speak it out loud. Syntax and grammatical stuff is the easy half for me.
Of all the problems I have, syntax is probably the greatest. Where to put those little words can be really confusing.
I can find something in every answer given above. Learning a new langauge is hard work ! I can follow the lessons, and completely understand it. A few days later, only half of it is still in my head, so I'm going back over it all again, in the hope of retaining more this time.
Newbie here, I'll chime in!
It's hard to say one thing that's most difficult... I agree with what's been mentioned above pretty much. Ser and estar were difficult but I think are getting easier. Right now I'm struggling with pronouns. Also whoever said the "little things," like the difference between por/para, when you use "que" (how many meanings can that one word have anyway!?) and other little words, and the accents -- I'm a good speller in English but not so much in Spanish, I'm realizing, because of when to use the accents! Haven't even gotten to the subjunctive lessons yet, oh dear...
OK I'll stop now before I get a mile-long list. Really I just need to spend more time with it daily, but somehow school + work doesn't always let me do that... 
hopefully better this summer!
I don't really have a problem per sé learning Spanish. Just like learning driving a car I have to go slow at first. I need to learn a bit about the car itself, wheels, engine, buttons, etc. (vocabulary) and also some about how it works all together (grammar & syntax.) Mountains can be moved with a spoon, it just takes tenacity, to not get discouraged, patience and will to succeed. A simple trick…while learning playing the guitar I was told to work only one hand at a time or my brains would get very confused (which they were already at birth LOL.) Dedicate some time to vocabulary, some time to grammar, syntax THEN and only then some time to all together. Being born French I have a great advantage. French has 14 tenses per verb so it's just a matter of learning the Spanish endings. Also most vocabulary is either the same or similar, I can actually make up word 50 to 60 percent of the time (entre=entre, piscina = piscine, etc.) and even the syntax a pretty much look alike. Maybe the only spot where I chock is pronunciation, saying reloj in the middle of a sentence still is difficult for me. What do we say here in the states? Even if at first you don't succeed, try, try again. So patience and dedication and someday we will all converse together in fluent Spanish.
