Words with Similar Meanings in Japanese

Right now I am studying the Core 2000 Japanese vocabulary list, which consists of the 2,000 most commonly used words in Japanese. I have been working my way through the vocabulary list for almost a month; I spend about 10-20 minutes or so a day studying the list via a flash card program called Anki, which is an excellent program for studying Japanese (or any other language for that matter). While many of the words that I've encountered are words that I was already very familiar with, there are other words that I did not know until I encountered them when studying the vocabulary list. Right now I have gotten through the first 513 words in the collection; many of those words I am now comfortable with. I should be finished studying the word list by the end of the summer.

One very interesting thing I discovered through my studies is that Japanese has a lot of words that are very similar to other words, but have a slight variation in meaning. For example, back when I was at Fujitsu, I learned the difference between 完了 and 終了, which both mean "to finish" but have slightly different connotations (the former implies that a task was completed, while the latter implies that something ended [but not necessarily completed], e.g., プロジェクトを完了しました [I completed the project] and プログラムが終了しました。[The program ended]).

Here are some additional groups that I noticed:

考える (to think, consider) vs. 思う (to think) vs. 検討する (to consider)

仕事 (work, job) vs. 作業 (work)

完了 vs. 完成 (both meaning "to finish, complete")

去年 vs. 昨年 (both meaning "last year")

変える vs. 変わる vs. 変化する (all meaning "to change")

大統領 vs. 社長 (both meaning "president")

開く　「あく」 vs. 開く　「ひらく」 (both meaning "to open"; notice that they are written exactly the same but pronounced differently)

行く　「いく」 vs. 行く　「ゆく」 (both meaning "to go"; same situation as above)

見せる vs. 示す (both meaning "to show")

閉める vs. 閉まる vs. 閉じる (all meaning "to close, to shut")

必要する vs. 要る (both meaning "to need")

It would be very interesting to see the differences between these words.