Did the Second Council of Orange deny the "certain efficacy" of grace? B. B. Warfield says in The Plan of Salvation [PDF]:


  Into the place of Pelagianism there stepped at once Semipelagianism; and when the controversy with Semi-pelagianism had been fought and won, into the place of Semi-pelagianism there stepped that semi-semi-pelagianism which the Council of Orange betrayed the Church into. ... The necessity of grace had been acknowledged as the result of the Pelagian controversy: its preveniency, as the result of the Semi-pelagian controversy: but its certain efficacy, its "irresistibility" men call it, was by the fatal compromise of Orange denied. ...  [After Orange it was not] any longer legally possible to ascribe salvation so entirely to the grace of God that it could complete itself without the aid of the discredited human will—its aid only as empowered and moved by prevenient grace indeed, but not effectually moved, so that it could not hold back and defeat the operations of saving grace.


But I've read through the canons of the Council of Orange and I don't see how he concludes that. Is he right?
Resistible grace in the Council of Orange

The conclusion to the council's canons says:


  After grace has been received through baptism, all baptized persons have the ability and responsibility, if they desire to labor faithfully, to perform with the aid and cooperation of Christ what is of essential importance in regard to the salvation of their soul.


This was the wording that I missed earlier. In other words, while God provides the initial impetus for salvation through grace, man is only saved thereafter "if they desire." This is clearly not consistent with "irresistible grace," or what Warfield terms "certain efficacy" of grace. Dialogist's answer is therefore more or less correct to say that what "troubled Warfield was that the Council of Orange ascribed any importance at all to man's free will," though Warfield, as a Calvinist, would affirm "free will" in a certain compatibilist sense.

Reformed objections to the Council of Orange

John Leith says in Creeds of the Churches, page 37:


  At the Council of Orange ... double predestination is repudiated and irresistible grace is omitted.


Similarly, Charles R. Biggs says in an article that the council "failed to articulate Augustine's teaching on predestination and irresistible grace." Biggs says in another article that "problems with the Synod of Orange" are the following:


  (1) The irresistibility of grace is not affirmed. (2) Predestination to evil is condemned. (3) The reception of grace is so bound to baptism that the sacramental quality of grace and the merit of good works are put in the foreground.


Herman Bavinck says in Reformed Dogmatics: Vol 1:


  The Synod of Orange ... accepted prevenient grace but did not decisively adopt irresistible grace and particular predestination. In the years following, not much Augustinianism was left intact."


So we can summarize the reformed objections to the council as follows:


Irresistible grace is omitted or denied (this is what the question was about)
Double predestination, which goes hand in hand with irresistible grace, is denied
Augustinian soteriology was weakened as a result and later diminished within the church until it was restored in the Reformation