What do you need to know about TF2?

The post that clown made, made me think. As I thought about how freaking big TF2 is, there's even the littlest tips and tricks like:

When sniper, your supposed to move periodically, as every kill you make they know where your at, and your mid range skills as sniper are terrible (I didn't learn that for 5 EFFING YEARS)

So really, if you have a question about this game post, post away.

I think the Problem is that a lot of people don't know how to use teamwork as an advantage. We are used to be able to fend for ourselves with weapons that can take down people from long ranges without any need of assistance.

Now I have horrible shot on PC games, but I like helping my teammates. That's why I fell In love with the medic class. =D