The United Reformed Church is a denomination in Britain that was formed out of a merger of the Congregationalist and Presbyterian denominations in 1972. What doctrinal compromises had to be made for the merger to succeed? In other words, what are/were the differences between Presbyterianism and Congregationalism, which beliefs and doctrines were carried forward into the URC and which had to be dropped?
The "Basis of Union" can be found here: http://www.urc.org.uk/what_we_do/the_manual/the_basis_of_union
For me, understanding any denomination usually hinges on two issues - salvation and governance. In my head, most any other distinctive could be chalked up to a peculiar practice or worship style, with its own concomitant theology. The comparatives are usually in salvation or governance.
So, from the beginning it should be said that Presbyterians descend from a Calvinist tradition (in Scotland), whereas Congregationalists and Puritans descend from a decided mixture of Calvinistic and Arminian traditions. In practice, I would assume that any already-in-the-minority free will positions that some Conrgegationalists held would have been further de-emphasized, but I couldn't find anything in the URC statement of belief to corroborate this.
In terms of church governance, Congregationalists stress the complete autonomy of the local church, although in practice many (like the Baptists) will come together in association in order to maximize their effectiveness. Presbyterians are very close to this - individual churches hire their own pastors, for example, but the power of the association and synod is a little greater. (Its nothing like say, a Catholic church, but there is some governance!) In practice, then, governance would have been a fairly easy compromise.