Its very difficult to take yourself back to medieval times, but you can look at the historical evidence that is available and come to some reasonable conclusions.
What comes to my mind is the seasonality of flavours, particularly sweetness. In northern Europe in winter you were shut up inside for months at a time because of the severe cold, with occasional brief forays outdoors (eg. to attend church, raid the dovecote for meat, and collect firewood - sticks and branches that fell from high up on trees because of the weight of snow). Your livestock was locked up too, in your house or in a barn (these were the animals remaining after you had butchered the rest of them in autumn because you didn't have enough hay in storage to keep them all alive until spring).
In this acrid, smoky household environment the taste of fresh ale with residual sweetness and little or no bitterness must have been a much anticipated joy, especially in contrast to the monotonous bitter and sour foods you were eating towards the end of winter - eg. rancid bacon and pickled vegetables. Come spring and summer there was sweetness in abundance, with honey, fruit and the sweet lactose in milk from your cows grazing on lush green pasture (as opposed to hay in winter).
The point I'm trying to make is that people were not chasing bitterness in their ale as we do now. Their preference would have been for sweetness because it was hard to get, particularly in winter. I disagree with those who propose that beer made today is the best beer ever made. For a start people did not drink beer until hops were used and that, in the long history of brewing, began five minutes ago. A lot of ale was drunk for a long time before and all the while hops were available.
If you had a time machine and took the finest of today's IPA back to medieval Europe they would probably spit it back in your face as horrible bitter piss. But people are very adaptable and tastes can be acquired and become rooted in culture if people are forced to adapt as happened with the Protestant Reformation.
As Tim has said earlier, the herbal mixture that was sold to be added to ale (gruit) up until the 1400s was a monopoly owned by the catholic monasteries. The rise of Protestantism was a reaction against the perceived excessive power and moral corruption within the Catholic church. Newly empowered protestant authorities ordered that gruit herbs be replaced with hops. It has been said that this was an economic attack on the catholic monopoly on gruit herbs. But consider also that the outlawing of gruit was perhaps an attempt by protestants to reduce vice and immorality.
The catholic church had many feast days. In the old church calendar there was some saint (great or small) to be commemorated just about every week. The protestant faith however frowned upon most of these feast days because of the drunkenness and debauchery that were exhibited at such events. In England in the early 1600s the extreme form of Protestantism known as Puritanism even banned the staging of plays because they were associated with excessive drinking and immorality.
The question I have is this. Was the dictate that gruit must end and hops be used to bitter ale (into beer) not just an economic attack on the catholic church's monopoly, but a move by protestant authorities to reduce the consumption of alcohol (and by association the sin of debauchery) by attempting to make beer less palatable? A similar move was apparently made in the time of Elizabeth I when it was decreed that lamb must be only be consumed with a bitter herb. This was at a time when England's economy was dependant on selling wool to the Low Countries, and it was a means to reduce the number of lambs being killed and being eaten by the peasants.
As I said people are very adaptable and rosemary soon became the bitter herb of preference to be taken with lamb, a tradition that survives to this day. And with beer the hop became not only accepted but sought after and bred into more palatable varieties. Such are the whims of fancies of the public taste.
