I often see people new to TF2Center taking a sub spot and not joining because they are not exactly sure what a sub button does. This can delay a lobby from starting or getting a sub for quite a while.
(And crush the hopes and dreams of people who've been waiting for a medic sub for 10+ minutes.)
I suggest there to be a 5 or 10 lobby requirement to see or take sub requests.
Just to make sure the people taking the sub request are familiar with Tf2Center and know how it works.