So, according to wikipedia (I know... but still it was the best source I could find)
Leading rabbis in Reform Judaism, Conservative Judaism, and Orthodox Judaism tend to hold that the death penalty is a correct and just punishment in theory, but they hold that it should not generally be used (or not used at all) in practice. In practice the application of such a punishment can only be carried out by humans whose system of justice is nearly perfect, a situation which has not existed for some time or never existed at all.
Why then did G-d instruct Jews in this manner? Do I have to interpret this as meaning that the old Jewish society is considered nearly perfect (which I have a hard time believing in the light of the mistakes that were made as described in the Torah)?