Scotus was the beginning of the decline of philosophy. He differed from the Angelic Doctor on many points.
Scotus, Opera Omnia (link courtesy: Ite ad Thomam)
also the Franciscan Archive's Scotus page
Scotus was a genius. And he single-handedly cleared the way theologically for universal acceptance of the true Catholic understanding of the Immaculate Conception. What led St. Thomas into his error on the matter was the inability that theologians had to reconcile freedom from Original Sin at the moment of conception with the necessity for all men to have been redeemed by Christ to be freed from Original Sin, an objection that Scotus dispatched with ease. Note, it's not that there wasn't universal belief in Our Lady's Immaculate Conception per se, or in principle, just that most believed that there had to be an instant during which Our Lady was in Original Sin in order for her to be freed from it; their objections were philosophical more than theological. Scotus' argument appears in the dogmatic definition made by Pius IX.
He differed with St. Thomas on philosophical subtleties that 99% of people have no clue about, and I actually like the fact that Scotus pushed a bit farther away from Platonism than where Aristotle ended up. He basically differed more with Aristotle than with St. Thomas. His articulation of the "formal distinction" was brilliant and not inconsistent with the same leanings in St. Thomas. Unfortunately, Scotus died relatively young and his followers did all kinds of violence to his writings, with many copies floating around that others took credit for and into which they interpolated some of their own thinking. Alas, sometimes the influence of William of Ockham (his rival) is confused with that of Scotus and Scotism is conflated with Okhamis, leading to this false charge against Duns. In any case, it almost appeared as if St. Thomas was a bit too enamored of "The Philosopher" (aka Aristotle), whereas Duns had no problem reconsidering some of Aristotle's thinking. Aristotle was not infallible as a Philosopher, and there's nothing un-Catholic about questioning some of his thinking. Where it came to theology both were solidly Catholic. In fact, it's that subtle push against Aristotelian thinking that enabled Scotus to push Catholic theology regarding the Immaculate Conception away from an artificial constraint.
It was because the 16th-century Scotists were ardent enemies of Renaissance humanism that these humanists coined the term that later became "dunce".