Why does tea brewed in a teapot always taste bad? Am I doing something wrong?

I have never had tea from a teapot that tasted good. Metal or ceramic, it didn't matter. The tea always tastes weak and I swear comes out with a hint of whatever the pot is made of.

I've always liked the idea of brewing a nice pot of tea, but aside from struggling to keep it warm, it just never tastes right. So I keep sticking to boiling water in a kettle and brewing in my cup.

Any advice?