It's somewhat of a fluke that the 1941-S is considered a "key" date. What happened was that when coin albums were popular in the 1950s and 1960s, the Walking Liberty half dollar series took two albums of space. The first album was the 1916 to 1940 issues and the second album was the 1941 to 1947 issues. The 1940 cut-off date was rather arbitrary and had little to do with anything significant about the series or the date. it was just the point at which the first album ran out out of space. The result was that the 1941-1947 album became a collectible set that was very popular with collectors of the day as it was much easier to complete than the earlier 1916-1940 album which contained the very rare early dates. So the 1941-S, which is indeed the rarest 1941-1947 in uncirculated and Gem condition, became the key date for the 1941-1947 "short set" (as it was called at the time). But the 1941-S is really not that rare. The 1940-S, for example, is much rarer. That said, the 1941-S remains an important coin and it is definitely scarcer than the other 1941-1947 issues.
For the 1941-S, like all of the 1940-1946 San Francisco Mint Walkers, strike is a problem. Specimens are often encountered, even otherwise superb Gem examples, that are very weakly struck on Ms. Liberty's left hand. Examples with a sharply struck hand are rare. Luster can be white and frosty (and I have seen many original rolls back in the day) and there are also many toned examples...some nicely so and some dark and dingy. Eye appeal is an issue so look for the white frosty or attractively toned examples.