The kanji. In opinion of most of the people I talk to, the hardest thing to learn in the Japanese language. There might me a lot of them, and as such, they appear impossible to master.
But knowing the kanjis can simplify learning vocabulary in Japanese. And it does simplify a lot.
The first thing is, that because there is a lot less letters in any given word, there is a lot less chance for misspelling. Hard to misspell a word with only one, two or even three letters.
The second thing is meaning. The words in the Japanese are usually logically arranged. In the logic of the Japanese people from the 10 century, sure, but a lot of that is similar to our.
For example, the word for pirate (海賊). The word for pirate literally means the burglar (海) of the sea (海).
Maybe something a little more common? What about the word for foreign (外人)? Foreign in Japanese literally means the person (人) from the outside (外).
What about the funeral (葬式)? The funeral means the ceremony (式), where we bury (葬) somebody.
Or the volcano (火山), that they are also known for. The volcano is the fire (火) mountain (山).
My favorite is probably the survival of the fittest (弱肉強食). The weak (弱) meat (肉) is eaten (食) by strong (強). The word itself explains the concept.
Or one of the funny ones. The word for warming bottle (魔法瓶). It means the magic (魔) way (法) bottle (瓶). It must have appeared that way to the Japanese people, when they first saw it.
There are always concepts, that are similar in cultures, but the different cultures understand it differently. Freedom (自由) in Japanese means the reason (由) for myself (自).
There are also some words, that don't make that kind of sense, but are still easier to remember. If simply because of asking, what did they meant with that. The example of that is carrot (人参), which means that person (人) is visiting (参).
I hope I made Japanese language at least a little less scary for the people. I hope that eventually, it will no longer be treated as one of the hardest languages, since it is really easy and fun.
If you have troubles with kanjis and not with words, you can read my column Why Learning Kanjis is Easy and Fun.