What aspect of learning Japanese has been the most challenging for you?

[JapanGuy]

It would have to be the never ending supply of kanji. No matter how many years you've studied, you will always come across words that use kanji in a way that you didn't know previously. The worst offender are city and town names! I can never get them right until someone familiar with the city tells me the correct reading. I sometimes feel if I would have spent the same amount of time studying Spanish or French that I have in Japanese, I'd be reading magazines and newspapers with no problem.

However, I'm not giving up! I continue to study and I want to pass the JLPT N1 later this year.

[AExAVF]

As of this writing, my biggest challenges to learning Japanese are listening as well as kanji.  By the time I enroll in Elementary Nihongo 2, I will have to study some more kanji in addition to the ones I have learned from my previous class.  Listening has always been my weakness.  But right now, I think my Nihongo language classes may have to take a back seat since I will be getting work in a few months time. 

[宇崎ちゃん]

My biggest struggle would be the part of kanji that have a whole sentence as its meaning (or at least, it feels like it).
The second would be the way their grammar translates.

Like:
Japanese: 読まなくちゃいけません。
Translation: I must read.
Literally: Unless you read, it does not go?

[Language-Student]

 

I think the difficulty must be "application" for me. Inside of the classroom, everything is fine, but where do I use it outside of the classroom, except online?

I was recommended to study Japanese on the basis that all the top scientific literature of the future will be in Japanese, so if you want to get ahead in your electronic engineering, you will need to study Japanese! Well, that was back in the early 1990's when Japan was quickly emerging as a techno-hub in electronic engineering design. So after four years of intensive Japanese, I managed to finally get a job at a time when we were going through "the recession we had to have". Unfortunately, the only language I needed to have studied in order to get ahead in my Automotive Electronics career, was German! Lesson number one: never listen to high school careers counselors and always go with your gut reaction! 

 

[Shounenbat]

Grammar and slang are the hardest for me.  Words and forms change so often that figuring out what some people say, especially on Youtube videos and in the comments section of Nico Nico Douga, is like learning an entirely new form of Japanese.  The kanji feels easy to me compared to the ever-changing words and grammar patterns.

[rainhero]

For me it is definitely Kanji, Hirigana, and Katakana. It's so hard for me to remember or understand that I mostly stick to romanization. Which I know in the end won't be good because I will need to be able to read the actual characters for Japanese reading, but I can't bring myself to study it.

[宇崎ちゃん]

@rainhero
Kanji can be very tough, but I honestly found hiragana and katakana the easiest part.
However, you may want to avoid the ancient trick of writing them down thousands of times, because that's not going to work (I tried it that way a year before I officially started studying/learning Japanese at all).