In 1949, when the novel was published, the year 1984 was far in the future. It therefore seemed possible the events Orwell described could actually come to pass in 35 years. Of course, to us, the year 1984 is decades in the past, making the work seem more fictive today than to early readers.
In many ways, the world Orwell and his readers lived in was different from our own and much closer to that of Oceania. For example, the London of Oceania, plagued with bombings, rubble, and people living in makeshift dwellings, was similar to the still bomb-damaged London of 1949. Even into the 1950s, some parts of the city waited to be cleared of rubble. Further, early readers, having lived through World War II, would have been quite familiar with the ration cards and food shortages described in the novel. In fact, rationing continued in England well past the end of the war.
A totalitarian government like the one running Oceania was also a reality in the world of 1949 in the form of Stalin's USSR. Thus, it wasn't farfetched for people to imagine a Stalin-like Big Brother ruling in England. People worried about a domino effect in which communism would take over the planet. To make the totalitarian threat even more real, Hitler and his totalitarian Nazi Germany had been defeated just four years before.
To the earliest readers of the novel, therefore, the world of deprivation and control that Orwell depicts would not have seemed as "other" as it does to us. The initial readers knew firsthand about hunger, rationing, bombing, living in substandard housing, lacking consumer goods, and the reality of totalitarian dictatorships. The novel, though still relevant today, must have seemed much more realistic, and therefore much scarier, in 1949 than it does to us.