What can I expect to pay for a taxi (two people with two pieces of luggage each) from Tegel Airport to our hotel in Potsdamer Platz.
According to taxifarefinder, 22 Euro (including a 10% tip) in medium traffic. I ran a "headcheck" on this calculator based on a recent trip that we took from our apartment near Schloss Bellevue to Tegel (2 of us with luggage - no extra fees) and it seemed spot on at least on the low traffic end (we traveled at 5 am).
Subject to traffic between €20 and €25 s mentioned above is correct. In Germany a reasonable tip for the driver is rounding up to the next full euro +1€... that's sufficient - unless the guy actually carried your luggage onto the 3rd floor or so. Berlin has another great taxi service: The free phone number 0800 CAB CALL. (Note: it's Zero 800, not one 800!) Especially if you wish to be able to pay by credit card calling a taxi through that service works great. They speak English. But at at the airport just get in the taxi that's next in line...