What Makes The Coin Valuable?

True basically what Conder101 said. Any coins prices are like anything else where one of the most important factors is popularity. If a coin is advertised, mentioned, discussed frequently, it becomes popular and the price goes up accordingly. A great example is the 1955 Lincoln Cent Double Die. In that series of coins there are numerous Double Dies but almost all collectors and dealers talk about the 1955 so that is the one worth most.
It is difficult naturally to make a normal, every day coin popular enough to warrant a high price, but those too do get some of that. This is why so many have everyday, normal coins slabbed. You take a 2002 Nickel in MS grades and have it slabbed and the price goes flying upward. If slabbed by a top discussed TPGS, such as PCGS, again the price goes up even higher.