The compromise of 1850 dealt with the land acquired from Mexico following the Mexican-American War and the Treaty of Guadalupe Hidalgo. The compromise included provisions such as California’s admission as a free state into the union, popular sovereignty for the rest of the territory of the Mexican cession, the abolition of the slave trade in the District of Columbia, and the fugitive slave law.
It was controversial because of the Fugitive Slave Law, which was stronger than the 1793 one. Many in the north continued to disobey it with personal liberty laws, and some, like Massachusetts, decided to nullify the law. This disobedience, piled with previous attempts at disobedience, by the North (Underground railroad) only further enraged the South and created more tension. The law was so crucial since it dealt with creating a more stringent law for the returning slaves to their owners, which was crucial to the South since slaves supported their way of life and economy.
It also could be considered controversial due to the political issue underlying it: representation in the Senate. If California had been admitted, it would tip the representation toward free states, which the South did not want to occur.