What issues did the Compromise of 1850 leave unsettled?

The Compromise of 1850 did not settle the question of
whether or not future states admitted after California or Texas would be slave or free
states, it merely provided the means by which it would be determined.  Popular
sovereignty was included in the Compromise, which meant that each state admitted after
1850 would also have a vote on slavery, where the majority would rule.  Of course, this
led to the disaster known as Bleeding Kansas, and no state was ever able to carry out
popular sovereignty.

In addition, the Compromise supposedly
strengthened the Fugitive Slave Act, which required law enforcement officials in the
North to arrest and return any slave found to have run to the North, under punishment of
heavy fines.  The compromise, however, provided no way to enforce such a rule, which was
always the problem with the Fugitive Slave Act.