My guidebook said Vienna had "comfortable, civilized" taxis, so I had my guard down coming out of Wien Hauptbahnhof. My personal experience did not fit this bill so I post this anecdote in the hope of warning future travelers!:
I took a taxi from the taxi stand at Wien Hauptbahnhof on May 2nd, 2018 around 14:30. The destination was Appartement-Hotel an der Riemergasse, a roughly 4 km trip on the shortest route.
Even though this was an official taxi stand with many taxis waiting in line, drivers seemed to be soliciting customers which seemed off to me. While I initially hesitated, I took the first taxi waiting as I had heavy luggage for 2 people.
Once in the taxi, I didn't see a meter and asked where it was, but the driver either didn't understand or intentionally deflected the question. At the end of the trip, something that looks like a meter emerged from behind the sun visor and the driver typed in 20.40 euros. Feeling probably scammed but tired from frayed nerves during the journey, I paid without argument but did demand a receipt and received it. No tip obviously.
When I showed my hotel receptionist the receipt, she raised her eyebrows but said different taxis seem to have different pricing that run the gamut (I paraphrase).
Since I have the receipt, I searched the web for how to file a complaint but couldn't find a place to do so online. I would like to do so if there is a way.