Leaves fell off:
If majority of the Arava leaves fell off the Arava, it is Pasul. One must be very careful regarding this matter, especially when inserting it into the binding of the Lulav, that this not cause the leaves to come off. [Thus it is advisable to check the Aravos daily to verify they still contain majority leaves. Likewise it is advised to purchase a number of sets of Aravos on Erev Sukkos, which can be used in a case of need throughout Sukkos.] If only minority of the leaves fell off it is valid.
Mitzvah Min Hamuvchar: Lechatchilah it is a Mitzvah Min Hamuvchar to use an Arava which still has all its leaves attached, if this is easily attainable.
 The reason: As it is no longer considered Hadar. [ibid]