The nature of kingship in the Middle Ages was very personal. For much of the Middle Ages (and remember we are encompassing over a thousand years that had a lot of dynamic changes so any generalizations are difficult) a king wasn't the head of a large bureaucracy. He was a man who swore to protect you in exchange for your allegiance to him. This is mostly going to based on Medieval England as it's my main area of expertise. But parallels can be drawn through out Europe.

In the early Middle Ages, the very start of the "Dark Ages", kingship was sometimes little more than being the leader of a warband. Men pledged their loyalty to a king because they respected him as a warrior. They believed a man could bring them glory and conquest and the booty that comes with it. As such it was imperative that a king fight in battle. It would be unthinkable for him not to. That was his whole function.

There wasn't much in the way of bureaucracy or other functions we think of as a head of state. They were military leaders first and foremost. This changed as time went on. While the bureaucracy would be limited from our view it really didn't take too long for it to come into existence. Kings established mints for their own coinage. They found more sophisticated ways to impose and collect taxes and fees. They became the head of the legal system and would adjudicate cases or respond to writs. But their role as leader of a warrior class was still the major function.

It also didn't take long for the kingship to become hereditary (in most places but not all). So, your right to rule was usually based on a family connection. But your ability as a leader in battle was still important.

A man like Henry II of England was a great king when it came to establishing many aspects of the common law that we know today. And he made justice slightly more uniform, he expanded laws, he reformed the currency. But he was respected and followed because he was a great battle commander. He fought in battles and won more than he lost. His son John had similar virtues when it came to legal matters and judicial reform. But he lost most of the French possessions and was regularly bested in battle. He was not a particularly respected monarch. Now, by this time not having respect as a war leader wasn't an automatic ousting or anything. But it definitely mattered as to the stability of your reign and how you were treated.

By the time we reach the "high" Middle Ages kingship had taken on many other dimensions. Leaders like Henry III were not very effective military leaders and while there was conflict with his barons and was forced to accept a ruling council of nobles he kept his throne through a very long reign. Just the fact that he could take the throne as a child shows that personal battle wasn't necessarily the only thing that mattered. However, a successful king was still often defined by his military prowess. Edward I and Edward III both had martial glory. Henry III and Edward II did not. Their respective reputations were shaped by that.

The culture of the time also demanded it of kings. While the concept of feudalism has come under justified fire, it's absolutely true that the system was largely set up to support a warrior class, at least in the early and through most of the high Middle Ages. Nobility essentially spent most of their lives training for battle for most of the Middle Ages. The very top of the pyramid not being prepared for or capable of leading men into battle would go against the entire setup of the culture.

Yes, it was dangerous. And kings did die in battle. Harold Godwinson the last Anglo-Saxon king of England, John of Bohemia (who was blind but still charged into battle), Richard III etc. Not to mention kings who died in ancillary ways associated with leading military campaigns like Richard I (gangrene from an arrow wound received during a siege) and Henry V (dysentery during a siege).

But, that being said, it wasn't as dangerous as you might imagine. Military battles weren't usually cases of everyone on the losing side being slaughtered. There are battles where that did happen and they are usually noted for the very reason that's it's atypical. But usually the result was a rout or the other side surrendering. Nobles would be valuable prisoners to ransom off. And, if feasible, they were often taken as prisoner rather than killed for this reason. Henry V famously had all his prisoners, including the nobility, killed at Angicourt and his men were angry about the lost ransom income because of it. A king would be well armored (of course) and surrounded by his personal guard who would be highly trained warriors. Even if they lost the battle and he wasn't able to escape (which would be rare) he would fetch a huge ransom. So, even when they were on the battle field the chances of death were relatively low. At least after the year 1000 or so, most kings didn't die in battle despite being present at them regularly.

In sum, it wasn't quite the same as Obama at the front of a tank column today. Commander and Chief is just one of a myriad of functions for the President, who is the head of a truly massive bureaucracy, whereas for much of the Middle Ages leading the military was the primary function of a king. Battles are even more unpredictable now with gunfire and explosives and taking place in a variety of locations whereas in the past shocks and twists happened but you usually knew the basics of what to expect and could plan for it. Our entire culture isn't set up to allow Obama's class to be effective warriors.