Frequently I find a few problems with sub slots, and think I have a few improvement suggestions.
1) If the game hasn't started yet, or only started a couple minutes ago, the sub icon could be something different, or could have the word "New" by it, to indicate that its a new lobby. This might encourage a lot more people to pick up that sub slot, especially medic slots.
2) If someone disconnects or rage quits in the last say 3-4 minutes for the map timer (usually 30 minutes) then a sub slot should not be requested even if the person is reported. I've joined several lobbies, one today even where I joined only to have the round end in 10 seconds. Why waste time with that?
3) I know the reliability system is probably still being worked on, but joining a sub slot I think should erase one prior rep that you have received. It would be like a way to quickly earn back your reputation.
Let me know your thoughts!