Well, since people got tired of the forced autobalance, Valve made it strictly voluntary. You can still use the old autobalance but, I'm getting the feeling Valve is silently teaching us a lesson.
Now joining games midround, well you could always form a party with your friends from the top and if you crash out you can rejoin, but I think they've disabled that feature or something. Kind of a problem indeed.
As of right now it does not (or very rarely) fill up the server halfway through a match, despite it claiming its looking for players. Because of there being no forced autobalance, this means one team will get stomped and make people leave, causing an uneven match with no fill in players. This has happened countless times to me and it seriously is ruining the game for me, as I used to love tf2.
For the party rejoin thing, i had to restart my game because of a glitch with the chat box preventing me from accessing menus or using my mouse. After I( got in i tried to join my friend while he was on dustbowl and it did not put me in the match for any of the 3 rounds, despite empty slots and saying i was queued to join.