Q:

Layover at Vienna for 6 hours - enough for some sightseeing?

In a week I'll be travelling from Sofia to Copenhagen, but with a transit through Vienna. I'll be arriving at 9:00 and my next flight is at 15:15, which makes a total of 6h15m. I thought this could be enough to grab a quick peak at the city and maybe one or two of the tourist attractions, since I've never been to Vienna before, but I heard it's certainly a place worth visiting.
Are these 6 hours enough to go in the city and check out at least one or two of the most popular tourist attractions, and which of them are closest to the airport?
(I'm an EU citizen, and I'll be travelling with no luggage, so hopefully I won't have to spend too much time at the airport dealing with luggage/passport control).

A:

I have transited through vienna around 15 times and happy to give you some tips on what to see during your 6 hour layoff. 

St.Stephens Platz church (start here. Its the city centre walk around for 2h you will see viennese operas, palace and shopping streets around. If you like cakes be sure to taste Sacher torta (shop not far from main opera) 
Schonbrunn palace (takes 2h minimum really nice with museum and zoo!)
Museum Quartier (walk around open space inside people will be chilling out in summer, of course if you are museum fan this is great). From Museum Quartier try to explore long shopping street Mariahilfer strasse (takes about 1 h)
"Hundertwasserhaus" is a very unique beautiful designed apartment houses worth a visit.

Vienna is very well connected by excellent metro system. All the places i mentioned above can be explored through metro/tram lines saving you time.
Last but not the least, have a coffee in viennese style. They will always serve coffee with a small glass of water to drink after the coffee (which i love it).

A:

The City Airport Train will take you (close to) the center in just 16 minutes. It is however a lot more expensive than the regular train going the same way in just 10 or so minutes more. Each of the two trains runs every 20-30 minutes, so the fastest option depends on when exactly you arrive at the train station.
From the terminal Wien Mitte-Landstraße you can either continue with the U3 subway line for 2 stations to get to Stephansplatz (the very heart of the city), or just walk there. All of the historical center is really nice and you can find nice and interesting places everywhere. If you want to see as many sights as possible on a short walk (1-2 hours if you want to see stuff), you can follow e.g., this route.
It's certainly possible to do this with a 6 hour overlay, but it will be quite stressful, especially if you need to pick up your luggage after arriving in Vienna. 

A:

Yes. For example, only 16 minutes to central Vienna, so sightseeing in that interval is viable but what to see there seems to be a matter of opinion so off topic here.

