Tosfos (Kiddushin 29) asks why a special limud is needed to exempt women from the mitzvah of milah. Why are women not exempt based on the blanket rule that women are exempt from any time bound mitzvah, any mitzvas aseh she'hazman gerama?
The Mahara"Ch Ohr Zarua answers Tosfos' question by suggesting that the exemption from zman gerama mitzvos applies where the mitzvah demands a specific act. The mitzvah of milah is not the act of cutting, but rather is the ongoing state of being nimol which is produced. The act of milah is just a means to an end, but not itself the essence of the mitzvah.
As we discussed last post, this question of whether the mitzvah of milah is the act of cutting or the effect produced of being in a state of being nimol may explain the dispute between the Rambam and Ra'avad as to whether one who is not nimol is chayav kareis every second until getting a milah, or chayav kareis only upon death. If the mitzvah is to be nimol, then every second of not being nimol is a new chiyuv kareis; if the mitzvah is the act of cutting, then until the moment of death when that act can no longer be performed there is no chiyuv.
YD commented that there may be a different way to learn this Rambam/Ra'avad. Perhaps the focus of their dispute is not the definition of the mitzvah of milah, but rather the definition of what a bitul mitzvah is -- is a deliberate delay considered a bitul mitzvah (in other words, bitul=lack of kiyum), or can we only say there is a bitul mitzvah when there is no longer a possibility of fulfilling the mitzvah? (See Chazon Ish to Kiddushin 29 who suggests a similar approach).
Just to clarify what I left out of the original post, RY Engel and others avoid this explanation because, for example, choosing to perform milah next week as opposed to today certainly displays a lack of zerizus and attention to the mitzvah, but it is, in their view, hard to classify that as a bitul of the mitzvah in an absolute sense.