I cannot agree with the above post that Caesar was assassinated because of his rejection of kingship; in fact he was assassinated because it was feared that he would proclaim himself King.
Caesar's greatest accomplishment was not so much his conquest of Gaul, which of course was significant; but his ability to secure rule of the entire Roman Republic for himself and to transform it into the Roman Empire. During the last days of the Republic, when Rome was racked by civil war, Julius Caesar raised his own private army using his own funds. In addition, he spent a great deal of money providing entertainment with the common folk, primarily gladiatorial fights and fights with wild animals. This made him immensely popular with the common people, and earned him membership in the First Triumvirate together with Pompey and Marcus Crassus. Caesar was to remain on the far side of the Rubicon River and leave Rome to Pompey (Crassus was killed during a slave revolt.) However, he was considered dangerous because of his popularity, and an attempt was made to remove him from power. In response, he marched on Rome with his army and took control. Pompey fled to Egypt.
Caesar is said to have muttered Jacta alia est ("the die is cast" when he crossed the Rubicon. Since that time, one is said to have "crossed the Rubicon" when one has reached the point of no return.
Caesar ruled under a provision of the Roman Constitution which provided for the creation of the office of Dictator during national emergencies. He married Cleopatra, Queen of Egypt, and set up colonies in Gaul, North Africa and Spain which he populated with veterans and poor folk so that they might have a new beginning. His popularity, however, made many fear that he would have himself crowned king--something the Romans never had--and he was accordingly assassinated.
The major accomplishments of Julius Caesar were military and political in nature. Basically, he had many military successes and those successes helped to allow him to become a political power in Rome.
Perhaps Caesar's most famous military act was his conquest of Gaul. This conquest greatly increased the size of the Roman empire (By "empire," I mean territories they conquered. Rome was not an empire in terms of its government at this point.), allowing Roman territory to reach all the way to the North Sea. This was hugely significant for Western and Northern Europe because it connected them to the culture and society of Rome.
Helped by this, Caesar was a significant Roman political figure. His greatest accomplishment is this regard was that he eventually came to be named dictator for life of Rome, having rejected the kingship. This, of course, led to his assassination.