You can't compare the originals to today...they are original, the one's that started it all.
While the effects are nothing like today's CG, the original King Kong set the bar for film making and effects. These guys did the best with what they had at the time to present a larger than life movie unlike anything anyone had ever watched in that time period. And if you can keep that in mind while watching this movie then you will see that it is more than a classic, it's an epic film.
There are films every so often that become the one film of it's time to set the bar for everyone after. The original King Kong pulls that off hands down and any aspiring filmmaker or special effects artist needs to study this film frame by frame.