What was preventing the populace from progressing in medieval times?

Do you know that the chimney was introduced to Europe in the 18th century? There had been use of the same principle in large commercial ovens and Roman baths but for some reason the idea to do the same thing for home heat didn't occur to them. Regular British homes didn't get them till 19th century.

Great leaps forward are not the norm. We are indoctrinated with the idea of steady progress, technologically and socially, but that's actually exceptional and was the product of massive investment (New Deal infrastructure projects, post-WWII rebuilding, moon project). Progress has actually stalled much as educational and infrastructure investment has declined. Things are glitzier but mostly refinement from what was invented during the 1940s-1970s. It is often said that War is a great stimulator of technology and economy, but the truth is that money is the stimulator; just more common to justify taxes and massive spending during times of war though we can do it anytime.
  Remember how cars 20 years old were "classics" and looked very different than the cars people were driving at the time? Not now, very little innovation going on. Even though we have the technology to automate and electrify our manufacturing, transportation, homes we relegate all implementation of those fundamental areas to private interests. Just like Europe after the fall of Rome, we have no central force to implement change. Government is supposed to but has only enforced mild regulation, not ordered change (equivalent to forcing the building of aqueducts and sewers).
  Homebuilding in the US got a small compromise update (regulation) in 1979 even though we have technology to make them many times more efficient at a price that would be negligible in a few years with energy savings. But that doesn't serve the energy/oil companies or the banks making real estate loans on cheaply built homes - to both builders and buyers.

Progress can be slow unless we work hard at it.

All answers so far are correct. There were advances in the Middle Ages but they revolved around the priorities at the time such as improvements in weapons, shipbuilding, metallurgy, and architecture (designs and methods for gothic cathedrals and palaces were developed during the Middle Ages and many construction projects on these types of buildings were started in the late Middle Ages). However, the Middle Ages did see a stagnation of scientific and philosophical ideas compared to the Roman and Greek era and the general population had less opportunity to evolve. The primary reasons for this stagnation were:

1. Religion. The Christian, Jewish, and Muslim religions were much more domineering in people's lives than previous religions during Roman and Greek eras and controlled every aspect of people's lives including scientific research, education, and philosophy, and were fearful of and hence restricted any idea that put into question its dominance or theology.

2. Feudalism. Small territories ruled by Lords which were always in conflict with each other were not condusive to trade, travel, peace, centralized authority, individual rights or prosperity, all of which tend to expand knowledge and technological and cultural advancement.

3. Population growth and plague. Cities in the Middle Ages had large growing populations but didn't have the sanitation that was customary in large Roman cities. As such, plagues and disease were common in the Middle Ages as well as recurring famine when weather conditions made for poor harvests unable to support the large city populations. During times of plague, disease, and famine, people tend to concentrate on survival rather than scientific advancement and distrust travel or travellers who they fear may bring the next plague.

Actually there's really no obvious answer. That's the beauty of history; it is subject to multiple interpretations.

First off, semantically, we have to define what it means to be "developing" or "progressing." It should be noted that certain scholars (especially Petrach, who is mentioned by name in this Wikipedia article) referred to the medieval period (roughly the 5th Century to the 15th Century) as "surrounded by darkness and dense gloom." Petrach felt that the era was marked by darkness: the so-called "Dark Ages." However, that's certainly not a matter of fact. Rather, it's a matter of historical interpretation; a question of historiography.

To offer a competing interpretation, Petrach wrote these comments in the early 14th Century, near the end but still solidly within the period that most historians now consider the Middle Ages. Surely a man living in the time period he is criticizing as being the "Dark Ages" is not unlike what Owen Wilson experienced in Midnight in Paris: the yearning or longing for a time thought to be more exhilarating, more artful, more plein de vie simply because one was not part of it.

Second, even taking Petrach's comment at face value, his characterization is at best extreme and at worst somewhat ignorant. The period was marked by significant technological, artistic, religious and economic advancements. For example, accurate mechanical time pieces (time pieces using an escapement mechanism--a technology that is still employed today in high end mechanical watches) were devised during this time period. The printing press was famously invented by Gutenberg in the mid-15th Century. Metal working was highly sophisticated during this period producing a variety of custom-made fully articulated armoured suits for both practical (military) purposes as well as for parades/ceremony. Not to mention the variety of weaponry including crossbows, swords, siege devices (trebuchet, catapaults) and cannons. See here and here.

But to answer your question: "what kept the people of Europe from advancing," a variety of factors influenced Petrach's characterization which has wound up as somewhat of a colloquialism. First, Europe was riddled (or "plagued," if you'll excuse the pun) with disease. The Black Death was estimated to kill somewhere between 30% to over 50% of the population of the European continent. Second, the theory that we as a people identify first with a nation or a country is a somewhat recent development in human history. In the Middle Ages people identified and pledged their allegiance to a "lord" or "master," a concept which formed the basis of feudalism. This sort of "governmental" power structure was not really conducive to getting a lot of things done at a macroeconomic or political level. "Lords" were highly concerned with consolidating land and power resulting in constant infighting, raids, sieges and cutting other people's heads off. See here.

Just that's a quick answer; by NO MEANS intended to be comprehensive. There's much more research to be done.