"Nein danke, ich möchte nicht."
"Das ist sehr nett, aber ich kann jetzt nicht."
"Nicht jetzt, danke. Gerne ein anderes Mal."  
"Nein danke, daran habe ich kein Interesse."

Direct communication is normal in Germany. You can say Nein directly, both in a personal and professional context. Of course, it is more polite to say danke along with Nein, and it's even better to combine it with a smile or give a reason in a friendly way. And the reverse is true: If someone offers you something (like a cup of tea) and you wish to accept it, you do not have to say no out of politeness the first time. A German will probably not ask you a second or third time.

"Es tut mir leid, aber ich sehe das anders."
"Ich habe da noch ein Problem, nämlich …"
"Kann ich dazu noch etwas fragen?"

The same goes for concerns. If you have doubts about something, you can express them, in a friendly way, even to a boss or teacher. It's advisable to formulate your concerns in a polite way. It's important that both sides maintain respect for one another.