1) I have received messages like that before. In both MMORPGs and FPS. When I play the Jade Quarry pvp mission in Guild Wars, I typically play a build called the Necro Bomber. It revolves around the concept that necromancers can sacrifice health to harm enemies. I rush the shrines, position myself appropriately and start sacrificing myself until I die and activate my Death Nova enchantment. It's an easy build to counter, but it often makes you the target of harassment by the enemy team.

Likewise, in TF2 I love playing the Pyro. However, the pyro has a lot of negative stigma attached to it. Many pyros are referred to as W+M1 players, simply running around setting everything on fire. Most of that hatred comes from when you either 1) Keep spy checking and killing spies and 2) manage to flank a group of enemies and set them all on fire.

It doesn't bother me and makes me laugh more often than not. If I'm feeling in the mood to chide the other players I usually tell them to "Cry some more".

2) It really depends on the map and game. I'm more often a run and gun kind of player. I like the feeling of kicking in the door and opening up on my enemies. As such, I tend to have kill/death ratios that are slightly above 1 or break even. My ratios used to be much higher, but I don't have the time to devote to FPSs as I used to.

I get bored "camping" for too long as a sniper and only play that class on certain maps.

I also like playing as a medic, but again I get bored if I play that too much. Which is annoying when you have teammates that go "We need medics!" constantly. After 3 maps, with multiple stages or rounds, of nothing but healing I need a change

3&4) No to both of these questions. It really depends on the mission and the map. For instance, if its a CTF or an Attack/Defend map, then the attackers have no right to complain about the defenders setting up and holding choke points. It's not cheating or noobish. And if you keep dying to the same sentry or sniper then you need to alter your tactics to compensate. Does the enemy have snipers? Maybe you should try counter sniping instead of trying to run from cover to cover. Sentry guns? Try using explosives from a distance or slip past them by another route. It's not the role of the defender to do stupid things so you suck less.

In TF2, Red frequently defends in those types of games. Therefore, it is reasonable and an anticipated tactic for RED to set up sentry guns, sniper nests, build uber-charges et cetera. While its fun to stomp the enemy every now and then, constant one sided battles get boring quickly.