Emphasis is important in theology. A church can believe in a doctrine but not emphasize it over a period of time. An example is 'sola gratia' - grace alone. Dr. R.C. Sproul is a very honest Reformed theologian and he says in his book "Faith Alone" that the Catholic Church has always fought to maintain that salvation is by the grace of God alone (though obviously not by faith alone, hence the Reformation). Louis Bouyer writes that if the Church focussed more on Grace Alone during the time of the Reformation, it is probable that the issues would have been settled. This is quite a claim of course, and easily debatable, but the general principle is that emphasis matters.
Jansenism is intriguing to me because I like it's emphasis, in only a few ways does it contradict Church teaching (possibly teaching limited atonement, and irresistible grace), but it places great emphasis on Election and the sovereign grace of God and faith in the work of Christ. It never denies that works justify or the nature of the communion or anything else like that (from the little of what I've read), but it does emphasize what I think at least is the 'best' of Protestantism and Catholicism.
Jansen himself wrote that if his doctrines were offensive to the Church that they should of course be disposed of and that he wished to be faithful to the Church, and I believe that is the point. The point is that emphasis needs to be reformed at times or shifted depending on the situation. Baptists for example believe in an eternally Hell for all believers which they will be conscious during, but I've never heard a sermon on it, because it's just alluded to briefly or like an 'unwritten law' that it exists. Thus if their churches start to doubt Hell, they should emphasize it.
Thus while I am not a Jansenist because it has been declared heretical, I definately agree with the emphasis and that the good parts of it's theology should be emphasized more. I don't know why people think Election is such a horrific doctrine, St. Paul, St. Augustine, St. Thomas Aquinas, and alot of others are in agreement that it is a beautiful thing. Active reprobation...is another story.