It is difficult to speak in generalities. The Puritans were a diverse group when you consider Anglican Puritans, Presbyterian Puritans, Congregational Puritans, Baptist Puritans; and the PCA is in some ways like the big-tent Republican Party (some PCA churches are like Episcopal churches; many are Evangelical; when I was in the PCA my congregation held to a cappella exclusive psalmody, which is very rare).
But it is generally agreed that the Westminster Standards of 1645-48 represents mature Puritan theology and to the extent that they differ from the PCA standards, one can discern major differences.
Specifically, the American revisions of 1788 and 1903 (relating to the civil magistrate and the Papal antichrist) of the Confession, which were adopted by the PCA, point to a significant difference in understanding the government of God in the church (the Pope is named as antichrist in the chapter on the church and this issue spills over into the question of the validity of Roman Catholic baptism as well; and the church's relation to the state). The Directory of Public Worship was revised early on in America too (before the inception of the PCA but this trajectory was followed by the PCA), to allow for the singing of uninspired hymns (the history of which is traced in Julius Melton's
Presbyterian Worship in America). Instrumental accompaniment has also been noted as a significant difference in the worship of the Puritans (it was nearly universally condemned by them) and the worship of the PCA (nearly universally condoned).
There are some in the PCA who are Puritan-minded, but broadly speaking, the PCA has diverged from the Puritan path in various ways, especially with respect to worship and ecclesiology.