After the Missouri Compromise met with problems in implementation, most notably in the balance of power between slave states and free states, the Kansas-Nebraska Act was drawn up to repeal the Missouri Compromise, to allow land for a transcontinental railroad and to create two new territories.
One of the major issues with the Act was the provision to allow settlers in the two new territories to vote on allowing or prohibiting slavery by Popular Sovereignty, instead of passing it as a bill through congress. Proponents argued that this would allow the South to continue slavery and the North to prohibit it without either side infringing on the other. Opponents pointed to the law as being entirely slanted towards the slave states, claiming that new territories should prohibit slavery entirely as that was the direction of the nation.
The bill's passage caused the creation of the New Republican Party, which strongly opposed slavery and so opposed the right of the settlers to allow slavery without Congressional approval, and divided the nation on ideological grounds. Voting on measures related to the Act were rife with fraud as each side tried to push their agenda, and violence broke out on the floor of the House before the Act was passed. John Brown's infamous massacre of pro-slavery settlers occurred, and slavery was the central issue in the Lincoln-Douglas Debates. With half the country advocating slavery and the other half opposing it, the seeds of the Civil War were planted.
One of the negative effects would be the fact that it permitted the expansion of slavery beyond the Southern states.