I echo many of the previous comments. Our lounge and 1 bedroom is south facing and the bricks outside heat up during the day and come the evening, (6pm onwards) it gets even hotter in there as it radiates back. We use fans and an air conditioner far more than a heater!
If its tall enough for a lift, ideally check that there are at least 2. Not such an issue if there are only a few flats, but if its a large block, many flats etc some places have at least 1 lift broken every week.
Try to visit at different times of the day- say weekday peak hour and also fri or sat evenings. You'll get an idea of how busy the street outside will be, road noise and movement in and out of the block at 8:30 on a monday. fri/sat evenings is generally when our upstairs neighbour starts their music blaring!
Our flat is very close to a tube line. In bed (we are 1st floor), you can hear it rumbling underneath. Not horrendous, but took a while to get use to it.
Since the 7/7 bombings, we now pay a terrorism charge in our council tax! I'm between zone 1-2.
Check the service charges
If you are able to speak to any neighbours, ask how accessible and responsive the freeholder is. Biggest issues with living there.
If you have a car, check the cost of residential parking in the borough.
Look around the streets and see how much parking is actually available (obviously if the block doesn't have parking on site)
Check your car complies with ULEZ. If you are within the congestion zone, there are charges for that too- but slightly reduced for residents
I believe that your home/contents insurance will be higher, if you live above a premises with cooking- cafe, kebab shop, pizzeria etc due to the added risk of fire. Fumes would also be an issue, so avoid being above or near any food prep shop.
Our flat is on a busy high street. Very convenient for shops, tube etc, BUT, I'm forever cleaning the windowsills of the rooms facing the road. Its a black, fine, soot which I assume is from buses and traffic. If I had a choice, I would have preferred being 1 street back because I'm sure the dust isn't healthy at all. I now have a super-dooper but expensive air purifier, but then the windows are closed and it get very hot!