I think that i should be able to switch characters mid game. Sometime i want to be a heavy or light and its annoying to back out to the main menu to change. If this is OP at least add the ability to switch characters in the lobby after a game.

Between rounds would be nice, but not during a match. If you could do this during a match then you would have people that had maxed all 4 classes with diff chars having an advantage over players with only one or two characters.
It is kind of lame having to choose between staying in a good lobby and swapping chars, but thats why my characters have 2 specializations.

My advice to you would be to either create a few well rounded characters, or to learn how to adapt your playstyle to whats required in each match. Its great to be able to stick with the class you want for an entire match having all the best skills but sometimes you have to switch to a class you dont have points in just to complete objectives.
These issues can be solved by finding a group of friends to roll with, or by just taking it for granted that sometimes you won't have the best loadout selected for your character. (Once again learning the intricacies of your main class will allow you to be dominate most situations)

then whats stoping a person for entering as a level 1 then switching to higher level

by not allowing higher ranks to enter.

your already in the game tho by the way he stated it