On the basis of an IFIP WG 2.3 meeting in Han-sur-Lesse (Belgium), Dijkstra expressed his misgivings about the overhead projector. He preferred the blackboard much more and the interaction it required between him and the audience. That is, he preferred to give "lectures", not "presentations". Instead of preparing everything in advance, he wanted room to improvise during his lectures. [EWD773]
It is also interesting to note that one-hour lectures were common during those days, compared to current-day "research presentations" of 15 minutes.