Is this a nice area to live in for a young couple (no kids)?!

Yes, Altstetten is convenient.

http://map.search.ch/altstetten-zuerich

i would recommend altstetten very much because:

-just 2 train stations from zurich main station
-good connections with tram number 2 and bus 31, very near to city
-apartments are less expensive than in the city
-you have all the shopping there, big migros and so on. otherwise city is very near
-nice forest and park. great for joggers or other sports in the green
-some clubs
-lots of companies near, maybe you work there and distance to work is very good

Just to add to the bit about shopping, the local shopping centre (Letzipark) is open Mon - Sat till 8pm, and the Coop local at Altstetten station is open 7 days a week till 11pm!

Other then that the main convenience is probably going to be the fact that you are two stops away from HB. If you travel regularly, there is a direct train to the airport once an hour.

All in all a pretty good location - though not if you want countryside.

Also the 3M Migros has most things, I have been living there isnce 1999.